Optical pickup

ABSTRACT

An optical pickup includes: a first projector for projecting a first light beam of a first wavelength so as to record and reproduce information with respect to an optical disk having a first light transmissive layer; a second projector for projecting a second light beam of a second wavelength longer than the first wavelength so as to record and reproduce information with respect to an optical disk having a second light transmissive layer; an objective lens common to the first and second light beams; and a diffraction optical element made of a lens with a diffraction grating and a refracting face and disposed in an optical path between the first and second projectors and the objective lens. The diffraction optical element is set to satisfy a predetermined equation. As a result, an optical pickup is realized that uses a single focusing means to focus light beams of different wavelengths, so as to record and reproduce information with respect to different kinds of optical disks (recording media) respectively having light transmissive layers of different thicknesses and respectively using different optimum wavelengths of light for reproducing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of, and claims priority to, U.S. patentapplication Ser. No. 10/625,709 filed Jul. 22, 2003, the disclosure ofwhich is herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to an optical pickup for recording andreproducing information with respect to a recording medium, andparticularly to an optical pickup for recording and reproducinginformation with respect to different types of recording media whoselight transmissive layers, from a light incident face to an informationrecording face, have different thicknesses, and whose optimum light beamwavelengths are different.

BACKGROUND OF THE INVENTION

Conventionally available are optical disk players (optical recording andreproducing apparatus) that can read recorded information from opticaldisks (optical recording medium), for example, such as DVD (DigitalVideo Disc) and CD (Compact Disc). DVDs currently available in themarket have the capacity as high as 4.7 GB, yet demand for higherdensity optical disks has been strong and there has been ongoing studyfor realizing such optical disks. It is well known that recordingdensity can be effectively improved by using light of a shorterwavelength for the reproducing light, and/or by increasing the NA(Numerical Aperture) of the objective lens.

In one optical pickup currently available for reproducing informationfrom a next-generation high-density optical disk, the numerical aperture(NA) of the objective lens has been increased from the conventionalDVD's 0.6 to 0.85, and a wavelength of 405 nm, shorter than theconventional DVD's 650 nm, has been selected for the reproducing light,so as to reduce the size of an aperture spot and thereby increaserecording density.

However, a problem of increasing the NA of the objective lens is that itdrastically increases the coma aberration that is caused when theoptical disk tilts, with the result that the focusing characteristic ofthe aperture spot may be impaired. Note that, as used herein, the term“coma aberration” refers to an aberration that is caused when the lightis focused on an axis other than the optical axis. The coma aberrationcaused by a tilt of the optical disk is proportional to the thickness ofthe light transmissive layer, from the light incident face to theinformation recording face, of the optical disk. Accordingly, aproportional increase of coma aberration with an increased NA of theobjective lens can be suppressed by reducing the thickness of the lighttransmissive layer of the optical disk. Based on this principle, it hasbeen proposed to reduce the thickness of the light transmissive layer ofthe next-generation high-density optical disk to 0.1 mm from theconventional DVD's 0.6 mm.

At the same time, the next-generation high-density optical disk needs toprovide compatibility with conventional DVDs and CDs, which are nowwidespread. That is, the optical disk player for reproducing thenext-generation high-density optical disk is required to reproduceconventional DVDs and CDs as well.

However, this is faced with one problem; namely, compatibility withdifferent kinds of optical disks is difficult to achieve when thewavelengths of light or thicknesses of the light transmissive layers aredifferent between different kinds of optical disks. As a rule, theobjective lens is designed for a specific thickness of the lighttransmissive layer of a particular type of optical disk, and a specificwavelength of light used therefor. Accordingly, in the event where thelight transmissive layers of the optical disks have greatly differentthicknesses or the optical disks use greatly different wavelengths,spherical aberration is caused on the aperture spot, impairing focusingcharacteristic of the aperture spot. Note that, as used herein, the term“spherical aberration” refers to the difference between a focal pointfor a paraxial ray near the center of the light beam and a focal pointfor a marginal ray distanced from the center of the light beam.

In view of this problem, there have been proposed optical pickups with aplurality of laser beam sources of different wavelengths and with asingle objective lens, whereby a laser beam is converged on aninformation recording face with a required numerical aperture.

For example, Japanese Publication for Unexamined Patent Application No.197717/2002 (published on Jul. 12, 2002) (Publication 1) discloses atechnique using an optical system in which the objective lens is madewith a diffracting face on a curved face of the objective lens, so as torecord and reproduce information with respect to three kinds of opticaldisks having light transmissive layers of 0.6 mm, 0.6 mm, and 1.2 mm,for which the wavelengths of 400 nm, 650 nm, and 780 nm are used,respectively. The objective lens with the diffracting face is designedsuch that the first order component of the diffracted light is used forthe light beam of each wavelength.

Another example is Japanese Publication for Unexamined PatentApplication No. 306261/2000 (published on Nov. 2, 2000) (Publication 2),which discloses an optical pickup device including a first light source,a second light source, a focusing optical system, and a compensatingoptical system. In this optical pickup device, the first light sourceemits a light beam with a wavelength of 650 nm. The second light sourceemits a light beam with a wavelength of 780 nm. The converging opticalsystem is configured to cause the light beam from the first light sourceto converge on an information recording face of a DVD without causingserious spherical aberration. The compensating optical system isdisposed between the second light source and the focusing opticalsystem. The compensating optical system is provided to suppress thespherical aberration that is caused when the focusing optical systemfocuses the light beam from the second light source focuses on aninformation recording face of a CD.

Yet another example is Japanese Publication for Unexamined PatentApplication No. 93179/2001 (published on Apr. 6, 2001) (Publication 3),which discloses a technique concerning an optical pickup for reproducingoptical disks having light transmissive layers of the same thickness,using different wavelengths of light. This technique uses two lightsources for respectively emitting a light beam (blue light) of 405 nmwavelength and a light beam (red light) of 650 nm wavelength. Adiffraction optical element and an objective lens that can focus theblue light on an optical disk having a light transmissive layer of 0.6mm thick are also used. In this technique, the light of eitherwavelength is incident on the diffraction optical element as a parallelray, and the second order component of the light diffracted by thediffraction optical element is used for the blue light, and the firstorder component of the light diffracted by the diffraction opticalelement is used for the red light, so as to attain sufficientdiffraction efficiency for both of these different wavelengths of light,and, at the same time, compensate for spherical aberration generated inthe red light.

Proceedings of the 63rd Annual Meeting of Applied Physics on “DVD/CDCompatibility Technique in Blue-ray Disc”, Naoki Kaiho et al., Fall,2002, No. 3, P.1008, Lecture Number (27p-YD-5) (Publication 4) disclosesa technique using an optical system including an objective lens and ahologram (diffraction element) that serves as a concave lens only for alight beam of 785 nm wavelength, the optical system recording andreproducing information with respect to three kinds of optical diskshaving light transmissive layers of 0.1 mm, 0.6 mm, and 1.2 mm, forwhich the wavelengths of 405 nm, 655 nm, and 785 nm are used,respectively.

The following describes the problems that are caused when the techniquedisclosed in Publication 1 is used for optical disks with lighttransmissive layers of different thicknesses, including anext-generation high-density optical disk (λ=400 nm, light transmissivelayer=0.1 mm), a DVD (λ=650 nm, light transmissive layer=0.6 mm), and aCD (λ=780 nm, light transmissive layer=1.2 mm).

As a rule, an optical pickup (compatible optical pickup) compatible withoptical disks of different recording densities uses an objective lensfor which aberration is compensated for with respect to the optical diskwith the largest recording density. Therefore, the compatible opticalpickup for the next-generation high-density optical disk, theconventional DVD, and the conventional CD uses an objective lens forwhich aberration is compensated for with respect to the next-generationhigh-density optical disk. The objective lens cannot be used directlyfor the DVD or CD whose light transmissive layers have differentthicknesses from that of the next-generation high-density optical disk,because in this case spherical aberration increases to the level whererecording or reproducing cannot be carried out.

One way to solve this problem when recording or reproducing DVD is tocompensate for the spherical aberration caused by the thicknessdifference of the light transmissive layers, by generating aberration inthe opposite direction. This can be carried out by causing the lightbeam to enter the objective lens as a diverging ray.

That is, in order to record or reproduce optical disks with lighttransmissive layers of different thicknesses, the light beams of therespective wavelengths are incident on the objective lens by varying thedegree of convergence and/or divergence of each light beam.

When a parallel ray of blue light (λ=400 nm) is incident on an objectivelens with an effective diameter of 3 mm to be focused on thenext-generation optical disk with a light transmissive layer of 0.1 mmthick, the degree of divergence for the red light (λ=650 nm) needs to beabout −0.03 in order to compensate for the spherical aberration causedby the thicker light transmissive layer of the DVD. Similarly, in thiscase, the degree of divergence for the infrared light (λ=780 nm) needsto be about −0.07 in order to compensate for the spherical aberrationcaused by the yet thicker light transmissive layer of the CD. Here, thedegree of convergence or divergence is an inverse of a focal length, andthe negative value indicates a diverging ray, and the positive valueindicates a converging ray.

Here, the red light and infrared light, with their large degrees ofdivergence of incident ray on the objective lens, greatly impair thefocusing characteristic of the light by causing coma aberration on theaperture spot on the optical disk when the objective lens shifts in theradial direction (direction substantially orthogonal to the optical axisof the incident light on the objective lens) during tracking or otheroperations. The impairment of focusing characteristic caused by radialshifting of the objective lens is more severe in CD because the degreeof divergence for the incident light on the objective lens is greater inCD.

With the objective lens having the diffracting face as disclosed inPublication 1, attaining diffraction efficiency of 100% for the firstorder component of one wavelength limits the diffraction efficiency forthe first order component of the diffracted light of the otherwavelengths, with the result that a desired level of high diffractionefficiency cannot be obtained. This brings about the problem of poorlight efficiency by a loss of light quantity. The loss of light quantitynecessitates a laser beam of higher power for the recording ofinformation in particular. Further, the diffracted rays of unnecessaryorders may enter the detector as stray light when reproducinginformation, with the result that the signal may be impaired.

When the technique disclosed in Publication 2 is used for the opticalpickup device for reproducing information from the next-generationhigh-density optical disk and the DVD, the optical pickup is providedwith an objective lens with a large numerical aperture. The objectivelens is made of glass of a high refractive index, and therefore hasstrong wavelength dependency. The strong wavelength dependency of theobjective lens poses a problem in that a focal point deviates greatly inthe presence of wavelength fluctuations caused by mode hopping orhigh-frequency superimposition, which cannot be followed by an actuator.

When the technique disclosed in Publication 3 is used for the opticaldisks with light transmissive layers of different thicknesses(next-generation high-density optical disk with a 0.1 mm thick lighttransmissive layer, and conventional DVD with a 0.6 mm thick lighttransmissive layer), and when the respective light beams of blue and redare incident on the diffraction optical element as parallel rays, theangle difference between the diffraction angle for the blue light andthe diffraction angle for the red light, which is required to compensatefor the spherical aberration caused by the large difference in thicknessof the light transmissive layers, must be increased to about 2° to 3°.The angle difference is related to the pitch of the diffraction gratingof the diffraction optical element, as shown by the graph of FIG. 35. Itcan be seen from FIG. 35 that the pitch of the diffraction grating needsto be as narrow as 3.5 μm to 4.5 μm in order to achieve the angledifference of about 2° to 3°.

Further, since the objective lens (infinite objective lens) is generallyoptimized for the blue light approaching from a point of infinity, theemergent ray from the diffraction optical element needs to be a parallelray. That is, a ray of blue light that is bent on the diffracting faceof the diffraction optical element needs to be refracted to a parallelray on entering the refracting face (face of the diffraction opticalelement on the side of the objective lens). This is also effective inpreventing aberration caused by misalignment of the diffraction opticalelement with the objective lens.

FIG. 36 represents a relationship between pitch of the diffractiongrating and curvature of the refracting face of the diffraction opticalelement, when a parallel ray of blue light incident on the diffractionoptical element emerges from the diffraction optical element as aparallel ray. Note that, the relationship represented in FIG. 36 isbased on a diffraction optical element in an optical pickup using anobjective lens with an effective radius of 2 mm. The refracting face ofthe diffraction optical element is spherical. It can be seen from FIG.36 that the curvature radius of the refracting face of the diffractionoptical element needs to be no greater than 2.2 mm in order to confinethe pitch of the diffraction grating from 3.5 μm to 4.5 μm.

However, given the fact that the effective radius of the objective lensis 2 mm, and that the effective diameter of the diffraction opticalelement is also 2 mm, the refracting face with a curvature radius of nogreater than 2.2 mm is substantially hemispherical, which is impossibleto fabricate or practically useless. The refracting face may be madeaspherical, but in this case the exceedingly small curvature makesfabrication of the diffraction optical element difficult. Even if it ispossible to fabricate, the on-axis focusing characteristic isundesirably increased to 0.018λ(rms) for all of the optical disks.

As a rule, the diffraction efficiency of a hologram (diffractionelement) for a given wavelength is determined by the depth of thediffraction grating. FIG. 2 is a graph representing a relationshipbetween depth of a diffraction grating and diffraction efficiency fordifferent wavelengths of light of different diffraction orders. In FIG.2, indicated by B0, B1, B2 are respectively diffraction efficiencies forthe zeroth order, first order, and second order components of thediffracted light of a light beam of 400 nm wavelength for thenext-generation high-density optical disk. Indicated by R0, R1 arerespectively diffraction efficiencies for the zeroth order and firstorder components of the diffracted light of a light beam of 650 nmwavelength for DVD. Ir0 and Ir1 are respectively diffractionefficiencies for the zeroth order and first order components of thediffracted light of a light beam of a 780 nm wavelength for CD.

FIG. 2 represents one application of the technique of Publication 1 inthe foregoing optical disk. As can be seen from the graph of FIG. 2according to one embodiment of the present invention, when the depth ofthe diffraction grating is set such that the first order component ofthe diffracted light of 780 nm wavelength yields higher efficiency thanthe other diffraction orders of the light of this wavelength, thediffraction efficiency for the diffracted light of a predetermineddiffraction order of the other wavelengths (zeroth order component ofthe diffracted light for the light beams of wavelength 405 nm and 650nm) falls below about 10%. Conversely, when the depth of the diffractiongrating is set such that the zeroth order component of the diffractedlight for the light beams of wavelength 405 nm and 650 nm yield higherefficiency than the other diffraction orders of the respectivewavelengths, the diffraction efficiency for the first order component ofthe diffracted light of 785 nm wavelength falls below about 10%. It istherefore practically impossible to set such a depth for the gratingthat the efficiency of light is increased for the all wavelengths oflight.

More specifically, when the depth of the diffraction grating is set forthe light beam of 405 nm wavelength, for which fabrication of a highpower laser is difficult, so as to increase the diffraction efficiency,i.e., the efficiency of using light, to, for example, 80% or greater,the efficiency of using the first order component of the diffractedlight of 780 nm wavelength decreases to 5% or less. In this case, theoptical pickup is unable to produce sufficient light quantity for therecording or reproducing of information with respect to CD.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical pickup thatcan (a) record or reproduce information with respect to recording mediarespectively having light transmissive layers of different thicknessesand respectively using optimum light beams of different wavelengths, (b)prevent light focusing characteristics from being severely impaired byradial shifting of an objective lens, and (c) suppress displacement of afocal point caused by wavelength fluctuations.

In order to achieve the foregoing object, an optical pickup of thepresent invention includes: a first light source that emits a firstlight beam of a first wavelength λ1; a second light source that emits asecond light beam of a second wavelength λ2 longer than the firstwavelength λ1; an objective lens that focuses the first light beam on aninformation recording face of a first recording medium having a firstlight transmissive layer, and focuses the second light beam on aninformation recording face of a second recording medium having a secondlight transmissive layer thicker than the first light transmissivelayer; and a diffraction optical element disposed in an optical pathbetween the first and second light sources and the objective lens, andincluding a diffraction grating and a lens with a refractive index n,wherein, when a distance between a diffracting face of the diffractiongrating and a peak of a lens face of the lens is a, a radius of thesecond light beam is R, and a pitch of the diffraction grating confinedby outermost rays of the second light beam passing through thediffraction grating is d,

said diffraction optical element is set so that m1 and m2, which arediffraction orders of the first and second light beams, respectively,satisfyƒ(d,m ₁)=ƒ(d,m ₂),where f(d, m_(x)), x being 1 or 2, is a function given by

${f\left( {d,m_{X}} \right)} = \frac{\left( {R - {a\;\tan\;\alpha_{X}}} \right)\sqrt{C_{X}^{2} + S_{X}^{2}}}{S_{X} - {C_{X}\tan\;\alpha_{X}} - {\sqrt{C_{X}^{2} - S_{X}^{2}}\tan\;\alpha_{X}}}$C_(X) = n cos  α_(X) − cos  β_(X) S_(X) = n sin  α_(X) − sin  β_(X)${{\sin\;\alpha_{X}} = \frac{m_{X}\lambda_{X}}{d}},$where α1 is an diffraction angle for m1-th order diffracted light forthe first light beam through the diffraction grating, β1 is an anglemade by a refracted ray of the m1-th order diffracted light through thelens with respect to an optical axis of the first light beam, α2 is andiffraction angle for m2-th order diffracted light for the second lightbeam through the diffraction grating, β2 is an angle made by a refractedray of the m2-th order diffracted light through the lens with respect tothe optical axis.

With this configuration, the first light beam is used for the firstrecording medium, and the second light beam is used for the secondrecording medium. The first light transmissive layer of the firstrecording medium is thicker than the second light transmissive layer ofthe second recording medium, and therefore the thickness of the secondlight transmissive layer causes aberration. The aberration can becompensated for by satisfying the foregoing equations. The presentinvention thus provides an optical pickup that can accurately focuslight beams on the light transmissive layers of the recording media, soas to record and reproduce information.

That is, by using the diffraction optical element that includes thediffraction grating and the lens so as to use light of diffractionorders satisfying the condition given by the foregoing equation, focusedlight spots can be formed to their diffraction limits on the recordingmedia respectively having light transmissive layers of differentthicknesses, even though the light sources with greatly differentwavelengths and the objective lens having a large numerical aperture areused. As a result, an optical pickup is provided that can record orreproduce information with respect to recording media respectivelyhaving light transmissive layers of different thicknesses andrespectively using different optimum wavelengths of light.

The described effect can also be obtained when the optical pickup withthe foregoing configuration is additionally provided with a collimatorlens that is provided between the first light source and the diffractionoptical element and between the second light source and the diffractionoptical element, and that causes the respective first and second lightbeams of the first and second light sources to be incident on thediffraction optical element as parallel rays.

It is preferable in the optical pickup of the present invention that thediffraction optical element includes the diffracting grating and thelens as an integral unit.

With this configuration, the number of components in the optical pickupcan be reduced.

It is preferable in the optical pickup of the present invention thatβ1=0, and β2>0.

With this configuration, when β1=0, the first beam is a parallel ray,which helps to improve shifting characteristics of the objective lens.Here, spherical aberration is caused in the second light transmissivelayer of the second recording medium. The spherical aberration, however,can be suppressed by making the second light beam a diverging ray withβ2>0.

It is preferable in the present invention that the diffraction order ofthe m2-th order diffracted light is equal to or lower than thediffraction order of the m1-th order diffracted light.

With this configuration, the m1-th order diffracted light and the m2-thorder diffracted light do not focus to a single point on the informationrecording face of the recording medium, so that the diffracted lightdoes not cause adverse effects in reading or recording.

It is preferable in the optical pickup of the present invention that thediffraction optical element satisfies m1=1, and m2=1. It is alsopreferable that the lens is a planoconvex lens with a spherical convexface, and the diffraction grating is formed on a plane face of theplanoconvex lens.

It is preferable in the optical pickup of the present invention that thediffraction optical element satisfies m1=1, and m2=0. It is alsopreferable that the lens is a planoconcave lens with an asphericalconcave face, and the diffraction grating is formed on a plane face ofthe planoconcave lens. Further, the diffraction grating shouldpreferably be provided on the side of the objective lens.

By satisfying these conditions, the optical pickup can be more suitablyrealized.

It is preferable in the optical pickup of the present invention that,when the diffraction order of first diffracted light is m1, thediffraction order of second diffracted light is m2, the pitch of groovedrings is d, and the sign of an angle created when a normal line of thediffracting face of the diffraction grating tilts toward the opticalaxis is positive, the diffracting face of the diffraction grating of thediffraction optical element satisfies

${{\sin^{- 1}\left( \frac{m_{1}\lambda_{1}}{d} \right)} - {\sin^{- 1}\left( \frac{m_{2}\lambda_{2}}{d} \right)}} > 0$with the diffraction order m1 of +1 for the first diffracted light, andwith the diffraction order m2 of 0 for the second diffracted light.

With this configuration, the diffraction order m1 of the firstdiffracted light is +1, and the diffraction order m2 of the seconddiffracted light is 0. Among different diffraction orders of thediffracted rays that satisfy the foregoing equation, the smallest valueof the diffraction order m1 is used for the first diffracted light andthe smallest value of the diffraction order m2 is used for the seconddiffracted light. In this way, diffraction efficiency can easily beimproved for both the first diffracted light and the second diffractedlight.

It is preferable in the optical pickup of the present invention that thediffracting face of the diffraction grating of the diffraction opticalelement has such a diffraction characteristic that the first and secondlight beams are diffracted toward the optical axis.

With this configuration, the diffraction characteristic of thediffracting face of the diffraction grating is such that the first andsecond light beams are diffracted toward the optical axis. This improvesdiffraction efficiency for the first diffracted light.

It is preferable in the optical axis of the present invention that thediffraction optical element has the diffracting face on an incident sideof the first and second light beams, and has a concave face on anemergent side of the first and second diffracted light, the diffractingface and the concave face having a common optical axis.

With this configuration, the diffraction optical element is made withthe diffracting face and the concave face. The concave face is providedto reduce on-axis chromatic aberration of the objective lens. Thus, byhaving the concave face in addition to the diffracting face,displacement of a focal point caused by wavelength fluctuations can besuppressed more effectively for the first diffracted light, as well asfor the second diffracted light.

It is preferable in the optical pickup of the present invention that theconcave face is aspherical.

With this configuration, the concave face of the diffraction opticalelement is made aspherical. By suitably shaping the concave face to havean aspherical shape, it is possible to reduce, more effectively than thespherical concave face, to reduce spherical aberration and form moredesirable light spots on the respective information recording faces ofthe two recording media respectively having light transmissive layers ofdifferent thicknesses.

The optical pickup of the present invention is suitably used in opticalrecording and reproducing apparatuses such as a DVD recording apparatusand a CD recording apparatus.

For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an optical system in an optical pickupaccording to one embodiment of the present invention.

FIG. 2 is a graph representing a relationship between depth anddiffraction efficiency of a diffraction optical element according to oneembodiment of the present invention.

FIG. 3 is an explanatory drawing showing action of the diffractionoptical element in the optical pickup.

FIG. 4 is a graph representing a relationship between first orderdiffracted light of a light beam of a first wavelength and diffractedlight of a light beam of a second wavelength, determining a curvatureradius in the diffraction optical element according to one embodiment ofthe present invention.

FIG. 5 is a graph representing a relationship between first orderdiffracted light of a light beam of a first wavelength and diffractedlight of a light beam of a second wavelength, determining a curvatureradius in the diffraction optical element according to anotherembodiment of the present invention.

FIG. 6( a) is a partial cross sectional view of an objective lens unit,explaining action of the diffraction optical element according toanother embodiment of the present invention when the first light beam isincident on the diffraction optical element.

FIG. 6( b) is a partial cross sectional view of an objective lens unit,explaining action of the diffraction optical element according toanother embodiment of the present invention when the second light beamis incident on the diffraction optical element.

FIG. 7( a) is a partial cross sectional view of an objective lens unit,explaining action of a diffraction optical element according to yetanother embodiment of the present invention when the first light beam isincident on the diffraction optical element.

FIG. 7( b) is a partial cross sectional view of an objective lens unit,explaining action of the diffraction optical element according to yetanother embodiment of the present invention when the second light beamis incident on the diffraction optical element.

FIG. 8 is a graph representing wavelength dependency of wavelengthaberration λrms, when the optical pickup of the embodiment is used toform a light spot on the information recording face of one of therecording media.

FIG. 9 is a schematic drawing showing a structure of an optical pickupaccording to one embodiment of the present invention.

FIG. 10( a) and FIG. 10( b) are cross sectional views of an objectivelens unit according to one Example of the embodiment of the presentinvention, showing how light beams are focused on optical disks with theobjective lens unit, in which FIG. 10( a) shows how the first light beamis focused on the first optical disk, and FIG. 10( b) shows how thesecond light beam is focused on the second optical disk.

FIG. 11( a) and FIG. 11( b) are graphs representing a relationshipbetween shift amount of the objective lens unit and wavefront aberrationwhen light beams are focused by the objective lens unit of FIG. 1, inwhich FIG. 11( a) represents the first light beam being focused, andFIG. 11( b) represents the second light beam being focused.

FIG. 12 is a graph representing changes in wavefront aberration as afunction of a wavelength of the first light beam when the objective lensunit of FIG. 10( a) and FIG. 10( b) focuses the first light beam on thefirst optical disk.

FIG. 13( a) and FIG. 13( b) are cross sectional views of an objectivelens unit according to one Example of the embodiment of the presentinvention, showing how light beams are focused on optical disks with theobjective lens unit, in which FIG. 13( a) shows how the first light beamis focused on the first optical disk, and FIG. 13( b) shows how thesecond light beam is focused on the second optical disk.

FIG. 14( a) and FIG. 14( b) are graphs representing a relationshipbetween shift amount of the objective lens unit and wavefront aberrationwhen light beams are focused by the objective lens unit of FIG. 13( a)and FIG. 13( b), in which FIG. 14( a) represents the first light beambeing focused, and FIG. 14( b) represents the second light beam beingfocused.

FIG. 15( a) and FIG. 15( b) are cross sectional views of an objectivelens unit according to Example 3 of the embodiment of the presentinvention, showing how light beams are focused on optical disks with theobjective lens unit, in which FIG. 15( a) shows how the first light beamis focused on the first optical disk, and FIG. 15( b) shows how thesecond light beam is focused on the second optical disk.

FIG. 16( a) and FIG. 16( b) are graphs representing a relationshipbetween shift amount of the objective lens unit and wavefront aberrationwhen light beams are focused by the objective lens unit of FIG. 15( a)and FIG. 15( b), in which FIG. 16( a) represents the first light beambeing focused, and FIG. 16( b) represents the second light beam beingfocused.

FIG. 17 is a schematic drawing showing a structure of an optical pickupaccording to one embodiment of the present invention.

FIG. 18 is a cross sectional view explaining the power of a diffractingface and the power of a refracting face of an diffraction opticalelement in the optical pickup of FIG. 17, and degrees of convergenceand/or divergence of a light beam entering and leaving the diffractionoptical element.

FIG. 19 is a graph representing a relationship between depth of thediffraction grating of the optical pickup of FIG. 17 and diffractionefficiency for the respective diffraction orders.

FIG. 20 is a graph representing a relationship between a degree ofconvergence and/or divergence of the first light beam and degrees ofconvergence and/or divergence of the second and third light beamsincident on the diffraction optical element of the optical pickup ofFIG. 17.

FIG. 21 is a graph representing a relationship between a degree ofconvergence and/or divergence of the first light beam incident on thediffraction optical element in the optical pickup of FIG. 17 andwavefront aberration caused in the first, second, and third light beams.

FIG. 22( a) through FIG. 22( c) are cross sectional views showing oneexample of degrees of convergence and/or divergence of light beams in anoptical pickup according to one embodiment of the present invention, inwhich FIG. 22( a) represents blue light, FIG. 22( b) represents redlight, and FIG. 22( c) represents infrared light.

FIG. 23( a) through FIG. 23( c) are graphs representing a relationshipbetween shift amount of objective shifting and wavefront aberration,comparing the Example shown in FIG. 22( a) through FIG. 22( c) with acomparative example.

FIG. 24 is a graph representing a relationship between wavelengthshifting of blue light and wavefront aberration according to the Exampleshown in FIG. 22( a) through FIG. 22( c).

FIG. 25( a) through FIG. 25( c) are cross sectional views showinganother example of degrees of convergence and/or divergence of lightbeams in the optical pickup according to the embodiment of the presentinvention, in which FIG. 25( a) represents blue light, FIG. 25( b)represents red light, and FIG. 25( c) represents infrared light.

FIG. 26( a) through FIG. 26( c) are graphs representing a relationshipbetween shift amount of objective shifting and wavefront aberration,comparing the Example shown in FIG. 25( a) through FIG. 25( c) with acomparative example.

FIG. 27 is a graph representing a relationship between wavelengthshifting of blue light and wavefront aberration according to the Exampleshown in FIG. 25( a) through FIG. 25( c).

FIG. 28 is a graph representing a relationship between power and minimumpitch of a refracting face of the diffraction optical element accordingto the Example shown in FIG. 25( a) through FIG. 25( c).

FIG. 29 is a graph according to the Example shown in FIG. 25( a) throughFIG. 25( c), representing a relationship between power of a refractingface of the diffraction optical element and wavefront aberration in bluelight when the shift amount of objective shifting is 200 μm.

FIG. 30( a) through FIG. 30( c) are cross sectional views showing oneexample of degrees of convergence and/or divergence of light beams in anoptical pickup according to one embodiment of the present invention, inwhich FIG. 30( a) represents blue light, FIG. 30( b) represents redlight, and FIG. 30( c) represents infrared light.

FIG. 31( a) through FIG. 31( c) are graphs representing a relationshipbetween shift amount of objective shifting and wavefront aberration,comparing the Example shown in FIG. 30( a) through FIG. 30( c) with acomparative example.

FIG. 32 is a graph representing a relationship between wavelengthshifting of blue light and wavefront aberration according to the Exampleshown in FIG. 30( a) through FIG. 30( c).

FIG. 33( a) through FIG. 33( c) are cross sectional views showinganother example of degrees of convergence and/or divergence of lightbeams in the optical pickup according to the embodiment of the presentinvention, in which FIG. 33( a) represents blue light, FIG. 33( b)represents red light, and FIG. 33( c) represents infrared light.

FIG. 34( a) through FIG. 34( c) are graphs representing a relationshipbetween shift amount of objective shifting and wavefront aberration,comparing the Example shown in FIG. 30( a) through FIG. 30( c) with acomparative example.

FIG. 35 is a graph representing a relationship between pitch of adiffraction grating of the diffraction optical element and diffractionangle difference of the diffraction optical element.

FIG. 36 is a graph representing a relationship between pitch of thediffraction grating and curvature of the refracting face of thediffraction optical element, when an incident parallel ray of blue lighton the diffraction optical element emerges from the diffraction opticalelement as a parallel ray.

FIG. 37( a) through FIG. 37( c) are cross sectional views showinganother example of degrees of convergence and/or divergence of lightbeams in the optical pickup according to the embodiment of the presentinvention, in which FIG. 37( a) represents blue light, FIG. 37( b)represents red light, and FIG. 37( c) represents infrared light.

FIG. 38( a) through FIG. 38( c) are graphs representing a relationshipbetween shift amount of objective shifting and wavefront aberration,comparing the Example shown in FIG. 33( a) through FIG. 33( c) with acomparative example.

FIG. 39 is a graph representing a relationship between wavelengthshifting of blue light and wavefront aberration according to the Exampleshown in FIG. 37( a) through FIG. 37( c).

FIG. 40 is a schematic drawing showing another Example of the opticalpickup of the present invention.

FIG. 41( a) is an explanatory drawing showing action of an objectivelens unit with a diffraction optical element having a concave face onthe side of a light source, when a first light beam is incident on theobjective lens unit in the optical pickup of the present invention.

FIG. 41( b) is an explanatory drawing showing action of the objectivelens unit in the optical pickup of FIG. 41( a), when a second light beamis incident on the objective lens unit.

FIG. 42( a) and FIG. 42( b) are schematic drawings showing anotherExample of the objective lens unit in the optical pickup of the presentinvention.

FIG. 43 is a graph representing a relationship between pitch of adiffraction grating of the diffraction optical element and diffractionangle difference of the diffraction optical element.

FIG. 44 is a graph representing a relationship between pitch of thediffraction grating and curvature of the refracting face of thediffraction optical element, when an incident parallel ray of blue lighton the diffraction optical element emerges from the diffraction opticalelement as a parallel ray.

FIG. 45 is a schematic drawing showing another Example of the opticalpickup of the present invention.

FIG. 46 is a schematic drawing showing one example of a recording andreproducing device with the optical pickup of the present invention.

DESCRIPTION OF THE EMBODIMENTS First Embodiment

FIG. 1 illustrates a schematic structure of an optical pickup 100 of thepresent embodiment. The description of the present embodiment will begiven based on the optical pickup 1 that is compatible with anext-generation high-density optical disk 14 a (first optical disk,first recording medium) and a conventional DVD 14 b (second opticaldisk, second recording medium).

The first optical disk uses blue light (first light beam) of a shortwavelength in the vicinity of 405 nm (first wavelength λ1), and has alight transmissive layer with a thickness t1=0.1 mm. The second opticaldisk uses red light (second light beam) of a long wavelength in thevicinity of 650 nm (second wavelength λ2), and has a light transmissivelayer with a thickness t2=0.6 mm.

The optical pickup 100 includes a semiconductor laser 1 a that emits afirst light beam 1 of the first wavelength λ1, and a semiconductor laser1 b that emits a second light beam 2 of the second wavelength λ2 longerthan λ1. The semiconductor laser 1 a and semiconductor laser 1 b areswitched (turned on) according to the type of target optical disk.

The optical pickup 100 further includes collimator lenses 2 a and 2 b,shaping optical systems 3 a and 3 b, and beam splitters 4 a and 4 b.Through the collimator lenses 2 a and 2 b, the first and second lightbeams respectively emerged from the semiconductor lasers 1 a and 1 bbecome parallel rays. The shaping optical systems 3 a and 3 b, such as ashaping prism, shape an ellipsoidal intensity distribution of the firstand second light beams into a substantially circular intensitydistribution. The beam splitters 4 a and 4 b pass the first and secondlight beams from the shaping optical systems 3 a and 3 b.

The shaping optical systems 3 a and 3 b are realized by a known opticalsystem, which may be a single triangular prism, a combined triangularprism, or two discrete triangular prisms. It should be noted that theshaping optical systems 3 a and 3 b are optional in the optical pickup100.

The semiconductor laser 1 a, the collimator lens 2 a, the shapingoptical system 3 a, and the beam splitter 4 a make up a first opticalsystem 16 a. The semiconductor laser 1 b, the collimator lens 2 b, theshaping optical system 3 b, and the beam splitter 4 b make up a secondoptical system 16 b.

The first and second light beams respectively emerged from the first andsecond optical systems 16 a and 16 b enter a dichroic prism 5 wheretheir optical axes merge. Leaving the dichroic prism 5, the first andsecond light beams travel through a common optical system.

In the common optical system, the first and second light beams passthrough a spherical aberration compensation system 6 and a ¼-wavelengthplate 8, and are reflected by a mirror 9 into an objective lens unit 13.

Entering the objective lens unit 13, the first and second light beamstravel through a wavelength-selective aperture filter 10, a diffractionoptical element 11, and an objective lens 12 in this order, and form asmall light spot on an information recording face of the first opticaldisk 14 a or second optical disk 14 b.

The spherical aberration compensation system 6, which is realized, forexample, by a beam expander or liquid crystal compensation element, isprovided to compensate for the spherical aberration caused by uneventhickness or other properties of the light transmissive layers of thefirst and second optical disks 14 a and 14 b.

In a configuration where the optical pickup 100 does not include theshaping optical systems 3 a and 3 b, the spherical aberrationcompensation system 6 may not be provided. The wavelength-selectiveaperture filter 10 controls aperture so that a numerical aperture NA1(0.85 to be specific) and a numerical aperture NA2 (0.6 to be specific)are obtained for the light beams of the first and second wavelengths λ1and λ2, respectively. Note that, the wavelength-selective aperturefilter 10, which is provided between the mirror 9 and the diffractionoptical element 11 in this embodiment, may be disposed in any otherposition, provided that the wavelength-selective aperture filter 10 isoperative as an integral unit with the diffraction optical element 11and the objective lens 12. Further, the wavelength-selective aperturefilter 10 may be realized by any other member, provided that it servesto control aperture.

Note that, the diffraction optical element 11 and the objective lens 12are held in place by a holder 17 (support member). The support membersupporting the diffraction optical element 11 and the objective lens 12prevents the diffraction optical element 11 and the objective lens 12from shifting relative to one another. In this way, impairment of thelight focusing characteristic caused by misalignment of the diffractionoptical element 11 with the objective lens 12 can be prevented.

The objective lens unit 13 includes a driving unit (driving means; notshown), so as to adjust a focal point of projected light on the opticaldisks 14 a and 14 b. That is, the driving unit is used to trackoscillations or rotation eccentricity of the optical disks 14 a and 14b.

The wavelength-selective aperture filter 10, the diffraction opticalelement 11, and the objective lens 12 are integrally provided as theobjective lens unit 13.

In addition to the foregoing light projecting optical system, theoptical pickup 100 further includes reproduced signal detecting opticalsystems 15 a and 15 b. The reproduced signal detecting optical systems15 a and 15 b are realized by a known optical system, and serve toreproduce a light spot control signal for auto focusing or tracking, oran information signal recorded in the optical disk.

The diffraction optical element 11 is made of glass or plastic, forexample. The diffraction optical element 11 has a diffraction gratinghaving concentrically grooved rings around the optical axis, or raisedorbicular bands formed by photolithography around the optical axis. Thediffraction grating is formed so that the cross section that cuts acrossthe plane including the optical axis is blazed (serrated) or stepped.The diffraction grating with the serrated or stepped cross section(serrated one in particular) is advantageous over other types ofdiffraction gratings because it offers higher diffraction efficiency.

The diffraction efficiency ηm of the blazed diffraction grating can begiven by Equation (7) below.

$\begin{matrix}{{\eta_{m} = {{\frac{1}{T}{\int_{0}^{T}{{A(x)}\exp\left\{ {i\;\phi\;(x)} \right\}{\exp\left( {{- i}\;\frac{2\pi\;{mx}}{T}} \right)}{\mathbb{d}x}}}}}^{2}},} & (7)\end{matrix}$where A(x) is the transmitted amplitude distribution, φ(x) is the phasedistribution, and T is the pitch of the grating.

Specifically, FIG. 2 represents diffraction efficiency of a diffractionoptical element when the diffraction grating is formed using a PC(polycarbonate) base. In FIG. 2, B0, B1, B2 respectively indicatediffraction efficiencies for the zeroth order, first order, and secondorder components of the diffracted light for the first light beam.Likewise, R0, R1, and R2 respectively indicate diffraction efficienciesfor the zeroth order and first order components of the diffracted lightfor the second light beam.

The efficiency of each diffraction order is determined by the depth ofthe diffraction grating. Accordingly, a sufficient recording orreproducing light beam can be obtained by suitably setting thediffraction order and the depth of the grating. In this way, an opticalpickup that can record and erase information requiring high power can berealized. In addition, the laser power can be reduced to suppress powerconsumption.

Further, the spherical aberration due to the thickness difference of thelight transmissive layers of the optical disks 14 a and 14 b isinversely related to the spherical aberration that is caused when adiverging ray is incident on the objective lens 12. Thus, in order tofocus a light beam on the recording layers of both the next-generationhigh-density optical disk and the DVD disk using the objective lens 12,the light beam of blue should preferably be incident on the objectivelens 12 as a parallel ray, and the light beam of red should preferablybe incident on the objective lens 12 as a diverging ray.

Therefore, it is preferable in the optical pickup 100 of the presentinvention that the diffraction optical element 11 is so designed thatthe first light beam of blue with a wavelength of 405 nm emerges fromthe diffraction optical element 11 as a parallel ray, and the secondlight beam of red with a wavelength of 650 nm emerges from thediffraction optical element 11 as a diverging ray.

Referring to FIG. 3, the following more specifically describes theobjective lens unit 13 of the optical pickup 100 to which the foregoingconditions apply. The objective lens unit is designed such that thelight beam that passes through the diffraction optical element 11 havingthe diffraction grating 11 a on a diffracting face of a lens portion 11b of a refractive index n is focused on the recording layer of theoptical disk 14 through the objective lens 12. The distance between thepeak of the lens portion 11 b and the diffracting face along the opticalaxis is denoted as a.

Note that, in FIG. 3, the broken line that focuses on the optical disk14 a via the diffraction grating 11 a and the objective lens 12 denotesthe first light beam, and the solid line that focuses on the opticaldisk 14 b via the diffraction grating 11 a and the objective lens 12denotes the second light beam.

Here, when the numerical aperture of the objective lens 12 is NA2 (0.6to be specific), the corresponding light beam (second light beam) of 650nm wavelength has an outermost radius R (radius R of the second lightbeam) when it passes through the diffracting face of the diffractionoptical element 11. In this case, when the first and second light beamsof the respective wavelengths pass through the diffraction opticalelement 11 within radius R, the beam angles created by the optical axisand the first and second light beams emerging from the diffractionoptical element 11 should respectively be set so that the first andsecond light beams are desirably focused on the recording layers of therespective optical disks. Specifically, when the diffraction order ofthe light beam of 405 nm wavelength is m1, and when that of the lightbeam of 650 nm wavelength is m2, the diffraction orders m1 and m2 allowfor the foregoing design when they satisfy Equation (8) below.r=ƒ(d,m ₁)=ƒ(d,m ₂)   (8),where r is the curvature radius, and f(d, m_(x)) is a function given byEquation (9) below.

$\begin{matrix}{{r = {{f\left( {d,m_{X}} \right)} = \frac{\left( {R - {a\;\tan\;\alpha_{X\;}}} \right)\sqrt{C_{X}^{2} + S_{X}^{2}}}{S_{X} - {C_{X}\tan\;\alpha_{X}} - {\sqrt{C_{X}^{2} - S_{X}^{2}}\tan\;\alpha_{X}}}}}{C_{X} = {{n\;\cos\;\alpha_{X}} - {\cos\;\beta_{X}}}}{S_{X} = {{n\;\sin\;\alpha_{X}} - {\sin\;\beta_{X}}}}{{{\sin\;\alpha_{X}} = \frac{m_{X}\lambda_{X}}{d}},}} & (9)\end{matrix}$where X is 1 or 2, and R is the radius of the second light beam on thediffracting face, corresponding to the numerical aperture NA2 of theobjective lens 12. In the diffraction grating 11 a of the diffractionoptical element 11, the diffraction orders of the first and second lightbeams are m1 and m2, respectively, and their diffraction angles are α1and α2, respectively. Further, the angles created by the optical axisand the first and second light beams emerging from the lens portion 11 bwhen the light beams desirably focus to form light spots on the opticaldisks (recording medium) is β1 and β2, respectively. The pitch ofdiffraction grating 11 a within radius R is d.

Specifically, the divergence angles β1 and β2 for canceling sphericalaberration vary depending on the shape of the objective lens 12. In thecase of the next-generation high-density optical disk and the DVD disk,β1 and β2 are approximately 0° and 2.5°, respectively, when the lensportion 11 b (base) of the diffraction optical element 11 is made ofpolycarbonate (PC) and when such a lens shape is selected, as will bedescribed later, that a parallel ray of blue light (first light beam)entering the objective lens 12 does not cause aberration on thenext-generation high-density optical disk. FIG. 4 and FIG. 5 represent arelationship between curvature radius r of the sphere and pitch d of thediffraction grating 11 a when radius R=1.1. FIG. 4 indicates the resultof calculation when the diffraction order of the blue light (405 nm) isthe first order, and FIG. 5 indicates the result of calculation when thediffraction order of the blue light (405 nm) is the second order. Here,a combination of diffraction orders that does not create a point ofintersection is unable to make diffraction optical element 11 thatenables the light beams to focus on both the next-generationhigh-density optical disk and the DVD disk. Table 1 below showscombinations of possible diffraction orders.

TABLE 1 DIFFRACTION ORDER PC COMBINATION OF RED BLUE DIFFRACTION BLUE:PARALLEL AND LIGHT LIGHT EFFICIENCY (%) RED: DIVERGENT 0 0 100 x 1 0 20∘ 2 0 5 ∘ 0 1 65 ∘ 1 1 78 ∘ 2 1 0 ∘ 0 2 10 ∘ 1 2 98 x 2 2 35 ∘

The divergence angle β2 for canceling spherical aberration is set to2.5° in the foregoing example. However, the combinations of diffractionorders realizing diffraction optical element 11 that enables the lightbeams to focus on both the next-generation high-density optical disk andthe DVD disk are effective as long as the angle of incidence is from 1°to 5°.

The following describes Examples of the optical pickup according to thepresent embodiment.

EXAMPLE 1

In this Example, the optical pickup 100 described in the foregoing FirstEmbodiment includes a diffraction optical element 11 which is realizedby a lens 116 that contains a diffraction grating 11 a and a convex face11 c, as shown in FIG. 6( a) and FIG. 6( b), the diffraction grating 11a being disposed on the side of a light source. It is assumed in thefollowing description that the base material (lens) of the diffractionoptical element 11 is polycarbonate (PC), and the diffracted rays of thefirst light beam (λ=405 nm) and the second light beam (λ=650 nm) usedare of the first order.

As shown in FIG. 6( a), a parallel ray of the laser beam (first lightbeam) of 405 nm wavelength incident on the diffraction optical element11 diffracts on the face of the diffraction grating 11 a in a directionof the first order diffraction (diverging direction). The first lightbeam then refracts as it passes through the convex face 11 c and emergesfrom the diffraction optical element 11 c as a parallel ray. Through theobjective lens 12, the parallel ray focuses on the recording layer ofthe optical disk (next-generation high-density optical disk) 14 a havinga 0.1 mm thick light transmissive layer. In this way, the optical pickup100 attains desirable light focusing characteristics.

As shown in FIG. 6( b), a parallel ray of the laser beam (second lightbeam) of 650 nm wavelength incident on the diffraction optical element11 c diffracts on the face of the diffraction grating 11 a in adirection of the first order diffraction (diverging direction). Thesecond light beam then refracts as it passes through the convex face 11c and emerges from the diffraction optical element 11 c as a divergingray. Through the objective lens 12, the diverging ray focuses on therecording layer of the optical disk (DVD disk) 14 b having a 0.6 mmthick light transmissive layer. In this way, the optical pickup 100attains desirable light focusing characteristics.

Here, the spherical aberration caused by the thickness of the lighttransmissive layer of the optical disk 14 b (DVD disk) can be suppressedwhen the diverging ray is incident on the objective lens 12. Sphericalaberration that cannot be compensated for this way can be compensatedfor by the aspherical portion of the diffraction grating 11 a.Evidently, the convex face 11 c may be aspherical by design, so as tomore effectively suppress aberration.

In order to prevent outer rays of the light other than thosecorresponding to the numerical aperture of 0.6 from focusing on theoptical disk 14 b having the light transmissive layer of 0.6 mm thick,the numerical aperture of the objective lens 12 is switched using thewavelength-selective filter 10 that passes light with a wavelength of405 nm but does not pass light with a wavelength of 650 nm.

In this Example, with the laser beam (first light beam L1; blue light)of 405 nm wavelength on the optical disk having the light transmissivelayer of 0.1 mm thick, the RMS wavefront aberration is only 0.002λ,which is sufficiently small. A sufficiently small RMS wavefrontaberration (0.002λ) is also obtained when the laser beam (second lightbeam L2; red light) of 650 nm wavelength is used for the optical diskhaving the light transmissive layer of 0.6 mm thick. This enables theoptical pickup 100 to sufficiently read out information signals from theoptical disks 14 a and 14 b.

In this Example, the diffraction optical element 11 has the diffractiongrating 11 a on the side of the light source and the convex face 11 c ison the side of the objective lens 12. However, the configuration of thediffraction optical element is not limited thereto. For example, thediffraction optical element may have the convex face on the side of thelight source, with the diffraction grating facing the objective lens.

Table 2 through Table 4 below show data that were obtained when theshape of the diffraction grating 11 a and the curvature radius of therefracting face of the diffraction optical element 11 were designed byan automated process. Specifically, the values in Tables 1 through Table4 below are the result of calculation on spherical aberration in thenext-generation high-density optical disk (wavelength of 405 nm, lighttransmissive layer of 0.1 mm thick) and DVD (wavelength of 650 nm, lighttransmissive layer of 0.6 mm thick). The objective lens used in Table 2through Table 4 below is so designed that blue light is optimallyfocused on the optical disk with the light transmissive layer of 0.1 mmthick.

TABLE 2 FACE CURVATURE FACE NUMBER RADIUS PITCH MATERIAL Next-GenerationHigh-Density Optical Disk (First Optical Disk) DIFFRACTION 1 INFINITY0.5 PC OPTICAL ELEMENT 2 −8.81 0.05 OBJECTIVE LENS 3 1.41 2.4LAH67_OHARA 4 16.37 0.1 DISK 5 INFINITY 0.1 PC 6 INFINITY 0.252482 DVD(Second Optical Disk) DIFFRACTION 1 INFINITY 0.5 PC OPTICAL ELEMENT 2−8.81 0.05 OBJECTIVE LENS 3 1.41 2.4 LAH67_OHARA 4 16.37 0.1 DISK 5INFINITY 0.6 PC 6 INFINITY 0.099382

TABLE 3 FACE NUMBER 5 FACE NUMBER 6 CONE FACTOR (K) −6.69E−01  −1.01E+01ASPHERICAL A 1.38E−02  4.95E−01 FACTOR B 4.04E−03 −9.12E−01 C 3.55E−04 5.38E−01 D 2.98E−04 −7.94E−02 E 9.33E−05  4.95E−01

TABLE 4 SECOND FACE DF 1 4.99E−02 DF 2 2.06E−04 DF 3 −5.13E−06  DF 40.00E+00 DF 5 0.00E+00

In Table 2 through Table 4, the face numbers 1 and 2 are the lightincident face and light emerging face of the diffraction optical element11, respectively. The face numbers 3 and 4 are the light incident faceand light emerging face of the objective lens 23, respectively. The facenumbers 5 and 6 are the surface and the information recording face ofthe optical disk. Note that, the face pitch in each row of the facenumber is the distance between the face of a given face number and theface of the next face number on the optical axis.

The phase function Φ(r) is given by Equation (10) below.

$\begin{matrix}{{{\Phi(r)} = {m\frac{\;{2\pi}}{\lambda}\left( {{{DF}\; 1r^{2}} + {{DF}\; 2r^{4}} + {{DF}\; 3\; r^{6}} + {{DF}\; 4\; r^{8}} + {{DF}\; 5\; r^{10}}} \right)}},} & (10)\end{matrix}$where m is the diffraction order, λ is the wavelength, r is the radiuswith respect to the optical axis, and DF1 through DF5 are factors.

In this Example (using the diffraction optical element 11 of Table 2through Table 4), with the laser beam (blue light) of 405 nm wavelengthon the optical disk having the light transmissive layer of 0.1 mm thick,the RMS wavefront aberration is only 0.002λ, which is sufficientlysmall. A sufficiently small RMS wavefront aberration (0.002λ) is alsoobtained when the laser beam (red light) of 650 nm wavelength is usedfor the optical disk having the light transmissive layer of 0.6 mmthick. This enables the optical pickup to sufficiently read outinformation signals from the optical disks 14 a and 14 b.

EXAMPLE 2

In this Example, the optical pickup 100 described in the foregoing FirstEmbodiment includes, as shown in FIG. 7( a) and FIG. 7( b), an objectivelens unit 113, instead of the objective lens unit 13 of the foregoingExample 1. That is, the optical pickup includes a diffraction opticalelement 111 which is realized by a lens 111 b made with a concave face111 c and a diffraction grating 111 a, the diffraction grating 111 abeing disposed on the side of a light source. The base material (lens)of the diffraction optical element 111 is polycarbonate (PC), and theface of the diffraction grating 111 a of the diffraction optical element111 satisfies Equation (11) below.

$\begin{matrix}{{{{\sin^{- 1}\left( \frac{m_{1}\lambda_{1}}{d} \right)} - {\sin^{- 1}\left( \frac{m_{2}\lambda_{2}}{d} \right)}} > 0},} & (11)\end{matrix}$where m1 is the diffraction order of the first diffracted light, m2 isthe diffraction order of the second diffracted light, and d is thegroove separation of the diffraction grating. It is assumed in thefollowing description that the diffraction order m1 of the firstdiffracted light M1 is +1, and the diffraction order m2 of the seconddiffracted light M2 is 0. That is, the first order component of thediffracted light is used for the first light beam (λ=405 nm), and thezeroth order component of the diffracted light is used for the secondlight beam (λ=650 nm).

As shown in FIG. 7( a), a parallel ray of the laser beam (first lightbeam L1) of 405 nm wavelength incident on the diffraction opticalelement 111 diffracts on the face of the diffraction grating 111 a in adirection of the first order diffraction (converging direction), andthen refracts in a diverging direction as it passes through the concaveface 111 c, and emerges from the diffraction optical element 111 as aparallel ray. Through the objective lens 12, the parallel ray focuses onthe recording layer of the optical disk 14 a having a 0.1 mm thick lighttransmissive layer. In this way, the optical pickup 100 attainsdesirable light focusing characteristics.

As shown in FIG. 7( b), a parallel ray of the laser beam (second lightbeam L2) of 650 nm wavelength incident on the diffraction opticalelement 111 refracts in the diverging direction as it passes through theconcave face 111 c, without being diffracted on the face of thediffraction grating 111 a (zeroth order diffraction). The diffractedlight then emerges from the diffraction optical element 111 as adiverging ray. Through the objective lens 12, the diverging ray focuseson the recording layer of the optical disk 14 b having a 0.6 mm thicklight transmissive layer. In this way, the optical pickup 100 attainsdesirable light focusing characteristics.

Here, the spherical aberration caused by the thickness of the lighttransmissive layer of the optical disk 14 b can be suppressed when thediverging ray is incident on the objective lens 12. Spherical aberrationthat cannot be compensated for this way can be compensated for by theaspherical portion of the diffraction grating. Evidently, the convexface may be aspherical by design, so as to more effectively suppressaberration.

In order to prevent an outer rays other than those corresponding to thenumerical aperture of 0.6 from focusing on the optical disk 14 b havingthe light transmissive layer of 0.6 mm thick, the numerical aperture ofthe objective lens 12 is switched using the wavelength-selective filterthat passes a wavelength of 405 nm but does not pass a wavelength of 650nm.

In this Example, with the laser beam (first light beam L1; blue light)of 405 nm wavelength on the optical disk having the light transmissivelayer of 0.1 mm thick, the RMS wavefront aberration is only 0.002λ,which is sufficiently small. A sufficiently small RMS wavefrontaberration (0.002λ) is also obtained when the laser beam (second lightbeam L2; red light) of 650 nm wavelength is used for the optical diskhaving the light transmissive layer of 0.6 mm thick. This enables theoptical pickup 100 to sufficiently read out information signals from theoptical disks 14 a and 14 b.

The foregoing described the case where the diffraction grating 111 a ofthe diffraction optical element is disposed to face the light source.However, the described effect can also be obtained according to thisExample when the concave face 111 c faces the light source. For example,the concave face 111 c of the diffraction optical element 111 may beprovided on the side of the diffraction optical element 111, as shown inFIG. 41( a) and FIG. 41( b).

In this case, as shown in FIG. 41( a), a parallel ray of the laser beam(first light beam) of 405 nm wavelength incident on the diffractionoptical element 111 diffracts on the face of the diffraction grating 111a in a direction of the first order diffraction (converging direction),and then refracts in a diverging direction as it passes through theconcave face 111 c, and emerges from the diffraction optical element 111as a parallel ray. Through the objective lens 112, the parallel rayfocuses on the recording layer of the optical disk 14 a having a 0.1 mmthick light transmissive layer. In this way, the optical pickup 100attains desirable light focusing characteristics.

As shown in FIG. 41( b), a parallel ray of the laser beam (second lightbeam) of 650 nm wavelength incident on the diffraction optical element111 refracts in the diverging direction as it passes through the concaveface 111 c, without being diffracted on the face of the diffractiongrating 111 a (zeroth order diffraction). The second light beam thenemerges from the diffraction optical element 111 as a diverging ray.Through the objective lens 112, the diverging ray focuses on therecording layer of the optical disk 14 b having a 0.6 mm thick lighttransmissive layer. In this way, the optical pickup 100 attainsdesirable light focusing characteristics.

Here, the spherical aberration caused by the thickness of the lighttransmissive layer of the optical disk 14 b can be suppressed to someextent when the diverging ray is incident on the objective lens 12.Spherical aberration that cannot be compensated for this way can becompensated for by the aspherical portion of the diffraction grating.Evidently, the convex face may be aspherical by design, so as to moreeffectively suppress aberration.

In order to prevent an outer rays other than those corresponding to thenumerical aperture of 0.6 from focusing on the optical disk 14 b havingthe light transmissive layer of 0.6 mm thick, the numerical aperture ofthe objective lens 112 is switched using the wavelength-selective filterthat passes a wavelength of 405 nm but does not pass a wavelength of 650nm.

Table 5 through Table 7 below show data that were obtained when theshape of the diffracting face of the diffraction optical element 111 andthe curvature radius and aspherical portion of the refracting face ofthe diffraction optical element 11 were designed by an automatedprocess. Specifically, the values in Tables 5 through Table 7 below arethe result of calculation on spherical aberration in the next-generationhigh-density optical disk (wavelength of 405 nm, light transmissivelayer of 0.1 mm thick) and DVD (wavelength of 650 nm, light transmissivelayer of 0.6 mm thick). The objective lens used in Table 5 through Table7 below is so designed that blue light is optimally focused on theoptical disk with the light transmissive layer of 0.1 mm thick. Further,the concave face is made aspherical in order to compensate for theaberration that is caused when the light of 650 nm wavelength isincident on the light transmissive layer of the optical disk.

TABLE 5 FACE CURVATURE FACE NUMBER RADIUS PITCH MATERIAL Next-GenerationHigh-Density Optical Disk (First Optical Disk) DIFFRACTION 1 INFINITY0.5 BK7_SCHOTT OPTICAL ELEMENT 2 10.9 0.05 OBJECTIVE LENS 3 1.46 2.5LAH67_OHARA 4 20.52 0.23 DISK 5 INFINITY 0.1 PC 6 INFINITY 0.15 DVD(Second Optical Disk) DIFFRACTION 1 INFINITY 0.5 BK7_SCHOTT OPTICALELEMENT 2 10.9 0.05 OBJECTIVE LENS 3 1.46 2.4 LAH67_OHARA 4 20.52 0.23DISK 5 INFINITY 0.6 PC 6 INFINITY 0.1

TABLE 6 SECOND FIFTH SIXTH FACE FACE FACE CONE FACTOR (K) 2.68E+01−6.69E−01  −1.01E+01 ASPHERICAL A −4.63E−03  1.38E−02  4.95E−01 FACTOR B2.31E−03 4.04E−03 −9.12E−01 C −8.22E−04  3.55E−04  5.38E−01 D 1.22E−052.98E−04 −7.94E−02 E 0.00E+00 9.33E−05  4.95E−01

TABLE 7 FIRST FACE DF 1 −3.56E−02 DF 2  2.15E−03 DF 3 −2.38E−03 DF 4 7.89E−04 DF 5 −5.17E−05

In Table 5 through Table 7, the face numbers 1 and 2 are the lightincident face and light emerging face of the diffraction optical element111, respectively. The face numbers 3 and 4 are the light incident faceand light emerging face of the objective lens 23, respectively. The facenumbers 5 and 6 are the surface and the information recording face ofthe optical disk. Note that, the face pitch in each row of the facenumber is the distance between the face of a given face number and theface of the next face number on the optical axis.

Indicated in Table 6 are aspherical factors for the respective faces.

In this Example (using the diffraction optical element 111 of Table 5through Table 7), with the laser beam (blue light) of 405 nm wavelengthon the optical disk having the light transmissive layer of 0.1 mm thick,the RMS wavefront aberration is only 0.002λ, which is sufficientlysmall. A sufficiently small RMS wavefront aberration (0.002λ) is alsoobtained when the laser beam (red light) of 650 nm wavelength is usedfor the optical disk having the light transmissive layer of 0.6 mmthick. This enables the optical pickup to sufficiently read outinformation signals from the optical disks 14 a and 14 b.

The diffraction optical element of the present Example includes theconcave face on the side of the light source and the diffraction gratingon the side of the objective lens. However, the effect of the presentinvention can still be obtained when the diffraction grating is providedon the side of the light source and the concave face is provided on theside of the objective lens.

The foregoing description in the Examples of the present invention wasmade based primarily on particular types of optical disks; namely, thenext-generation high-density optical disk (light transmissive layer of0.1 mm thick; using blue light (λ=405 nm)) and DVD (light transmissivelayer of 0.6 mm thick; using red light (λ=650 nm)). However, the presentinvention is not just limited to these particular types of optical diskswith particular thicknesses of the light transmissive layers, orparticular wavelengths of light used therefor. The present invention isapplicable to a wide variety of optical disks with light transmissivelayers of different thicknesses or using different wavelengths of light.

As described, in one aspect of the invention, the present invention is acompatible optical pickup that uses a common objective lens to form afocused light spot on a first recording medium and a second recordingmedium, the first recording medium having a light transmissive layer ofthickness t1 across the light incident face and the informationrecording face, and the first recording medium recording or reproducinginformation with a focused light spot that is formed using a first lightsource emitting a first light beam of a first wavelength and anobjective lens with a numerical aperture NA1, the second recordingmedium having a light transmissive layer of thickness t2 (t2>t1) acrossthe light incident face and the information recording face, and thesecond recording layer recording or reproducing information with afocused light spot that is formed using a second light source emitting asecond light beam of a second wavelength longer than the firstwavelength and using a portion of the objective lens corresponding to anumerical aperture NA2 (NA2<NA1). The optical pickup includes adiffraction element and a lens with a refractive index n, thediffraction element being disposed in an optical path between the firstand second light sources and the objective lens, and the optical pickupuses such a combination of m1 and m2 that satisfy Equation (12) below.ƒ(d,m ₁)=ƒ(d,m ₂)   (12),where X is 1 or 2, and f(d, m)Equation (12) is a function given byEquation (13) below.

$\begin{matrix}{{{f\left( {d,m_{X}} \right)} = \frac{\left( {R - {a\;\tan\;\alpha_{X\;}}} \right)\sqrt{C_{X}^{2} + S_{X}^{2}}}{S_{X} - {C_{X}\tan\;\alpha_{X}} - {\sqrt{C_{X}^{2} - S_{X}^{2}}\tan\;\alpha_{X}}}}{C_{X} = {{n\;\cos\;\alpha_{X}} - {\cos\;\beta_{X}}}}{S_{X} = {{n\;\sin\;\alpha_{X}} - {\sin\;\beta_{X}}}}{{{\sin\;\alpha_{X}} = \frac{m_{X}\lambda_{X}}{d}},}} & (13)\end{matrix}$where a is the distance between a face of the diffraction element and apeak of a lens face, R is the radius of the second light beam on aportion of the diffracting face corresponding to the numerical apertureNA2 of the objective lens, m1 is the diffraction order of the firstlight beam diffracted on the diffraction element, m2 is the diffractionorder of the second light beam diffracted on the diffraction element, α1and α2 are diffraction angles, β1 and β2 are the angle made by theoptical axis and the light beam through the objective lens when thelight beam forms a desirable focused light spot on the recording medium,and d is the pitch of the diffraction element within radius R.

In this way, by using the diffraction optical element that includes thediffraction grating and the lens so as to use diffraction orderssatisfying the condition given by the foregoing equation, focused lightspots can be formed to their diffraction limits on the recording mediarespectively having light transmissive layers of different thicknesses,even though the light sources with greatly different wavelengths, andthe objective lens having a large numerical aperture are used. As aresult, an optical pickup is provided that can record or reproduceinformation with respect to recording media respectively having lighttransmissive layers of different thicknesses and respectively usingdifferent optimum wavelengths of light.

FIG. 8 represents wavelength dependency of wavefront aberration λrms inthe optical pickup 100 of the described embodiment and in the opticalpickup device of the foregoing conventional example, where the former isindicated by solid line A and the latter is indicated by broken line B.It should be noted here that the wavefront aberration λrms for eachwavelength is the smallest wavefront aberration λrms that provides thebest focusing with a given wavelength.

The wavefront aberration λrms gradually increases as the firstwavelength λ1 of the first light beam increases or decreases from 405nm. It can be seen from FIG. 8 that the rate of increase of thewavefront aberration λrms is smaller in the optical pickup 100 of thepresent embodiment than that in the optical pickup device of theconventional example. In can be seen from this that the optical pickup100 of the present embodiment has lower wavelength dependency.

The first and second light sources 1 a and 1 b are realized by asemiconductor laser device as described above, and therefore causewavelength fluctuations by mode hopping for example. Wavelengthfluctuations also occur when there is high-frequency superimposition.Such wavelength fluctuations cause displacement of a focal point whichcannot be tracked by the actuator driving the objective lens 12. Theoptical pickup 100 of the present embodiment generates less wavefrontaberration than the optical pickup device of the conventional example inresponse to wavelength fluctuations caused by mode hopping orhigh-frequency superimposition. This suppresses displacement of a focalpoint due to wavelength fluctuations, thus desirably forming a lightspot even in the presence of wavelength fluctuations.

Further, in the present Embodiment, displacement of a focal point due towavelength fluctuations in the first light beam can also be suppressedeven when the optical pickup is specifically designated for thenext-generation high-density optical disk. That is, the foregoing effectis still effective even when the optical pickup is made with componentsthat are designated only for the first light beam.

The first light beam that is emitted from the first optical system 16 aas a parallel ray can emerge from the diffraction optical element 11also as a parallel ray when the power Φ of the diffraction opticalelement 11 satisfies the following equationΦ=Φ_(D)+Φ_(L)=0where Φ_(D) is the power of the diffracting face of the diffractiongrating 11 a, and Φ_(L) is the power of the refracting face of thediffraction grating 11 a. In this way, the aberration caused bymisalignment with the objective lens 12 can be suppressed. In this case,the diffraction optical element 11 may be disposed anywhere between thefirst optical system 16 a and the objective lens 12.

The present Embodiment adopts the diffraction optical element with theconcave face on the side of the light source and the diffraction gratingon the side of the objective lens. This is advantageous in terms ofmanufacture in increasing the pitch of the diffraction grating and thecurvature of the concave face. However, not limiting thereto, theeffects of the present invention remains the same even when thediffraction optical element has the diffraction grating on the side ofthe light source and the concave face on the side of the objective lens.

The optical pickup of the present embodiment as shown in FIG. 1 may bemodified to provide an optical pickup as illustrated in FIG. 40. Theoptical pickup shown in FIG. 40 has the same structure as the opticalpickup of FIG. 1, except that the ¼ wavelength plate 8 and the sphericalaberration compensation system 6 are switched in position in the opticalpickup, and that the diffraction optical element 11 and thewavelength-selective filter 10 are switched in position in the objectivelens unit 13.

Second Embodiment

FIG. 9 illustrates a schematic structure of an optical pickup 200 of thepresent embodiment. The description of the present embodiment will begiven based on the optical pickup 200 that is compatible with anext-generation high-density optical disk 14 a (first optical disk,first recording medium) and a conventional DVD 14 b (second opticaldisk, second recording medium). The first optical disk uses blue light(first light beam) of a short wavelength in the vicinity of 405 nm(first wavelength λ1), and has a light transmissive layer with athickness t1=0.1 mm. The second optical disk uses red light (secondlight beam) of a long wavelength in the vicinity of 650 nm (secondwavelength λ2), and has a light transmissive layer with a thicknesst2=0.6 mm. The optical pickup 200 includes a semiconductor laser 1 athat emits the first light beam 1 of the first wavelength λ1, and asemiconductor laser 1 b that emits the second light beam 2 of the secondwavelength λ2 longer than λ1. The semiconductor laser 1 a andsemiconductor laser 1 b are switched (turned on) according to the typeof target optical disk.

The optical pickup 200 further includes collimator lenses 2 a and 2 b,shaping optical systems 3 a and 3 b, and beam splitters 4 a and 4 b.Through the collimator lenses 2 a and 2 b, the first and second lightbeams respectively emerged from the semiconductor lasers 1 a and 1 bbecome parallel rays. The shaping optical systems 3 a and 3 b, such as ashaping prism, shape an ellipsoidal intensity distribution of the firstand second light beams into a substantially circular intensitydistribution. The beam splitters 4 a and 4 b pass the first and secondlight beams from the shaping optical systems 3 a and 3 b.

The shaping optical systems 3 a and 3 b are realized by a known opticalsystem, which may be a single triangular prism, a combined triangularprism, or two discrete triangular prisms. It should be noted that theshaping optical systems 3 a and 3 b are optional in the optical pickup2.

The semiconductor laser 1 a, the collimator lens 2 a, the shapingoptical system 3 a, and the beam splitter 4 a make up a first opticalsystem 16 a. The semiconductor laser 1 b, the collimator lens 2 b, theshaping optical system 3 b, and the beam splitter 4 b make up a secondoptical system 16 b.

The first and second light beams respectively emerged from the first andsecond optical systems 16 a and 16 b enter a dichroic prism 5 wheretheir optical axes merge. Leaving the dichroic prism 5, the first andsecond light beams travel through a common optical system.

In the common optical system, a spherical aberration compensation system6 varies the extent to which the first and second light beams convergeor diverge, according to the type of light beam (first light beam orsecond light beam). Then, the first and second light beams pass througha ¼-wavelength plate 8, and are reflected by a mirror 9 into anobjective lens unit 213. Namely, the optical pickup 200 of the presentembodiment includes the objective lens unit 213, as shown in FIG. 9,instead of the objective lens unit 13 or 113 of the First Embodiment.

Entering the objective lens unit 213, the first and second light beamstravel through a wavelength-selective aperture filter 210, a diffractionoptical element 211, and an objective lens 212 in this order, and form asmall light spot on an information recording face of the first opticaldisk 14 a or second optical disk 14 b.

The spherical aberration compensation system 6, which is realized by abeam expander, is provided to compensate for spherical aberration causedby the thickness difference or other properties of the lighttransmissive layers of the first and second optical disks 14 a and 14 b.In addition, the spherical aberration compensation system 6 serves aslight beam control means for controlling the extent to which the firstand second light beams converge or diverge, as noted above.

In a configuration where the optical pickup 2 does not include theshaping optical systems 3 a and 3 b, the spherical aberrationcompensation system 6 may not be provided. In this case, the collimatorlenses 2 a and 2 b may be used to vary the extent of convergence ordivergence of the first and second light beams. Further, any otherelement may be used to vary the extent of convergence or divergence ofthe first and second light beams.

The wavelength-selective aperture filter 210 controls aperture so that anumerical aperture NA1 (0.85 to be specific) and a numerical apertureNA2 (0.6 to be specific) are obtained for the light beams of the firstand second wavelengths λ1 and λ2, respectively. Note that, thewavelength-selective aperture filter 210, which is provided between themirror 9 and the diffraction optical element 211 in this embodiment, maybe disposed in any other position, provided that thewavelength-selective aperture filter 210 is operative as an integralunit with the diffraction optical element 211 and the objective lens212. Further, the wavelength-selective aperture filter 210 may berealized by any other member, provided that it serves to controlaperture.

The wavelength-selective aperture filter 210, the diffraction opticalelement 211, and the objective lens 212 are integrally provided as theobjective lens unit 213. The objective lens unit 213 as an integral unitis movable relative to other optical systems of the optical pickup 200.In this way, a focused light spot can accurately follow the oscillationsof the information recording face of the first and second optical disksor rotation eccentricity of the information track of the first andsecond optical disks.

In addition to the foregoing light projecting optical system, theoptical pickup 200 further includes reproduced signal detecting opticalsystems 15 a and 15 b. The reproduced signal detecting optical systems15 a and 15 b are realized by a known optical system, and serve toreproduce a light spot control signal for auto focusing or tracking, oran information signal recorded in the optical disk.

The diffraction optical element 211 is made with a convergingdiffraction grating 211 a and a diverging planoconcave lens 211 b, so asto suppress the wavefront aberration caused by wavelength fluctuationsand obtain desirable light focusing characteristics against wavelengthfluctuations.

The diffraction optical element 211 is made by forming the diffractiongrating 211 a on a plane face of the planoconcave lens 211 b, so thatthe number of components can be reduced. Alternatively, the diffractionoptical element 11 may be composed of two optical elements made up of adiffraction element and a lens.

Further, the objective lens unit 213 may be realized using diffractionoptical element 211 that includes a diffraction grating on therefracting face of a translucent lens on the other side of the planeface.

The planoconcave lens 211 b of the diffraction optical element 211 ismade of glass or plastic. The diffraction grating 211 a of thediffraction optical element 211 is formed on the plane face of theplanoconcave lens 211 b by forming concentrically grooved rings aroundthe optical axis, or by forming raised orbicular bands around theoptical axis by photolithography. The diffraction grating 211 a isformed so that the cross section that cuts across the plane includingthe optical axis is blazed (serrated) or stepped. The diffractiongrating with the serrated or stepped cross section (serrated one inparticular) is advantageous over other types of diffraction gratingsbecause it offers higher diffraction efficiency.

It should be appreciated that the same effect can be obtained when thediffraction optical element 211 has the diffraction grating 211 a on theconcave face of the planoconcave lens 211 b. In this case, alignment ofthe concave face with the diffraction grating becomes easier, whichhelps to form the concave face and the diffraction grating more easily.

The diffraction orders of the light diffracted on the diffractiongrating 211 a of the diffraction optical element 211 are selected foroptimum efficiency, such that the optical pickup 200 records andreproduce information using the second order component of the diffractedlight for the first light beam (λ=405 nm), and the first order componentof the diffracted light for the second light beam (λ=650 nm).

The optical pickup 200 causes at least one of the first and second lightbeams to enter the diffraction optical element 211 as a converging rayor diverging ray, i.e., a non-parallel ray. That is, the first andsecond light beams enter the diffraction optical element 211 atdifferent converging or diverging angles. This attains a diffractionangle difference of about 0.5° to about 1.5° between the first andsecond light beams, which is necessary for the compensation of thespherical aberration caused by the large thickness difference betweenthe respective light transmissive layers. As a result, the pitch of thediffraction grating can be made wider, and the curvature radius of theconcave face can be made greater, enabling the diffraction opticalelement 211 to be fabricated more easily.

As the term is used herein, the “converging or diverging angle” is theangle made by the optical axis and the outermost edge of the light beamon a cross sectional plane including the optical axis of the light beam.The converging or diverging angle is negative for a converging ray, andis positive for a diverging ray.

Incidentally, as shown in FIG. 42( a), an objective lens unit 120′ maybe realized with a diffraction optical element 122′ that includes adiffraction grating 122 b′ on a refracting face (concave face) S2 of atranslucent planoconcave lens 122 a′ on the other side of a plane faceS1.

The planoconcave lens 122 a′ of the diffraction optical element 122′ ismade of glass or plastic. The diffraction grating 122 b′ of thediffraction optical element 122′ is formed on the plane face S1 of theplanoconcave lens 122 a′ by forming concentrically grooved rings aroundthe optical axis, or by forming raised orbicular bands around theoptical axis by photolithography. The diffraction grating 122 b′ shouldpreferably be formed, as shown in FIG. 3, that the cross section thatcuts across the plane including the optical axis is blazed (serrated).The diffraction grating with the serrated cross section is advantageousover other types of diffraction gratings because it offers higherdiffraction efficiency. Alternatively, an objective lens unit 120″ maybe formed that includes a diffraction grating 122 b″ so that the crosssection that cuts across the plane including the optical axis isstepped, as shown in FIG. 42( b). The diffraction grating 122 b″ withthe stepped cross section is advantageous over other types ofdiffraction gratings because it offers higher diffraction efficiencynext to the diffraction grating 122 b′ with the serrated cross section.

It should be noted that the effect of the diffraction optical element122″ shown in FIG. 42( b) remains the same even when the diffractiongrating 122 b″ is formed on the concave face S2 of the planoconcave lens122 a″ in the manner shown in FIG. 42( a). In this case, the concaveface S2 and the diffraction grating 122 b″ can be aligned more easily,making it easier to form the concave face S2 and the diffraction grating122 b″.

The following describes Examples of the optical pickup 200 according tothe present embodiment.

EXAMPLE 3

In this example, as shown in FIG. 10( a) and FIG. 10( b), an objectivelens unit 213 is prepared so that a first light beam L1 (λ=405 nm)enters the objective lens unit 213 as a parallel ray (i.e. at aconverging or diverging angle θ of 0°) and the objective lens unit 213uses the second order component of the light diffracted by a diffractionoptical element 211, and that a second light beam L2 (λ=650 nm) entersthe objective lens unit 213 as a diverging ray (i.e., at a converging ordiverging angle θ of 1.5°) and the objective lens unit 213 uses thefirst order component of the light diffracted by the diffraction opticalelement 211. An aspherical lens is adopted for an objective lens 12. Thediffraction optical element 211 has a concave face that is alsoaspherical.

When the first light beam enters the diffraction optical element 211 asa parallel ray, the light diffracted in the second order diffractiondirection (in the converging direction with respect to the optical axis)by a diffraction grating 211 a of the diffraction optical element 211 isrefracted in the diverging direction by a planoconcave lens of thediffraction optical element 211 and enters the objective lens 212 as aparallel ray. The diffracted light is then focused by the objective lens212, passes through a light transmissive layer (thickness of 0.1 mm),and forms a small light spot on an information recording face, therebyobtaining a desirable light focusing characteristic.

When the second light beam L2 enters the diffraction optical element 211as an diverging ray, the light diffracted in the first order diffractiondirection (in the converging direction with respect to the optical axis)by the diffraction grating 211 a of the diffraction optical element 211is refracted in the diverging direction by the planoconcave lens 211 bof the diffraction optical element 211 and enters the objective lens 212as a diverging ray. The diffracted light is then focused by theobjective lens 212, passes through a light transmissive layer (thicknessof 0.6 mm), and forms a small light spot on the information recordingface, thereby obtaining a desirable light focusing characteristic.

By thus causing the first light beam L1 and the second light beam L2 toenter the diffraction optical element 211 as a parallel ray and adiverging ray, respectively, the diffraction grating 211 a and theplanoconcave lens 211 b can be designed to have a pitch and a concaveface that satisfy a converging or diverging angle θ of 0° for the secondorder component of the diffracted light for the first light beam L1entering the objective lens 212, and a converging or diverging angle θof 2° to 3° for the first order component of the diffracted light forthe second light beam L2 entering the objective lens 212 (i.e., adiffraction angle difference of 2° to 3° is achieved between the secondorder component of the diffracted light for the first light beam L1 andthe first order component of the diffracted light for the second lightbeam L2).

Table 8 through Table 11 below show data concerning the respective facesof the objective lens 212 and diffraction optical element 211 sodesigned. In the Tables, the face number 0 indicates a virtual lightsource. The face numbers 1 and 2 are the light incident face and lightemerging face of the diffraction optical element 211, respectively. Theface numbers 3 and 4 are the light incident face and light emerging faceof the objective lens 212, respectively. The face numbers 5 and 6 arethe surface and the information recording face of the optical disk. Notethat, the face pitch in each row of the face number is the distancebetween the face of a given face number and the face of the next facenumber on the optical axis.

Table 10 indicates aspherical factors for the respective faces, andTable 11 indicates the factor for each term of Equation (14)representing a phase difference function Φ(r) for the diffracting face.In Equation (14), m is the diffraction order, λ is the wavelength, r isthe distance from the optical axis, and DF1 through DF5 are factors.Further, in Tables 10 and 11, −2.2E-03 denotes −2.2⁻³. The same notationis used throughout the Tables.

TABLE 8 NEXT-GENERATION HIGH-DENSITY OPTICAL DISK (FIRST OPTICAL DISK)FACE CURVATURE FACE NUMBER RADIUS PITCH MATERIAL VIRTUAL LIGHT 0INFINITY INFINITY SOURCE DIFFRACTION 1 INFINITY 0.5 BK7_SCHOTT OPTICALELEMENT 2 8.319 0.5 OBJECTIVE LENS 3 1.465 2.5 LAH67_OHARA 4 20.516 0.1DISK 5 INFINITY 0.1 PC 6 INFINITY 0.23

TABLE 9 DVD (SECOND OPTICAL DISK) FACE CURVATURE FACE NUMBER RADIUSPITCH MATERIAL VIRTUAL LIGHT 0 INFINITY 50 SOURCE DIFFRACTION 1 INFINITY0.5 BK7_SCHOTT OPTICAL ELEMENT 2 8.319 0.5 OBJECTIVE LENS 3 1.465 2.5LAH67_OHARA 4 20.516 0.1 DISK 5 INFINITY 0.6 PC 6 INFINITY 0.10

TABLE 10 FACE NUMBER K A B C D E ASPHERICAL 2 0.0E+00 −2.2E−03  −2.3E−03−1.4E−04  1.4E−04 0.0E+00 FACTOR 4 −6.9E−01  1.3E−02  3.1E−03 2.6E−055.2E−04 −6.7E−05  5 7.2E+02 4.5E−01 −1.2E+00 1.5E+00 −9.4E−01  5.0E−01

TABLE 11 FACE NUMBER DF1 DF2 DF3 DF4 DF5 PHASE 1 −3.E−02 6.E−04 8.E−046.E−05 −5.E−05 DIFFERENCE FACTOR

[Equation 14]

$\begin{matrix}{{\Phi(r)} = {m\;\frac{2\pi}{\lambda}\left( {{{DF}\; 1\; r^{2}} + {{DF}\; 2\; r^{4}} + {{DF}\; 3\; r^{6}} + {{DF}\; 4\; r^{8}} + {{DF}\; 5\; r^{10}}} \right)}} & (14)\end{matrix}$

FIG. 11( a) and FIG. 11( b) show changes in wavefront aberration λrms onan image surface with respect to an amount of shift of the objectivelens unit 213 in the optical axis direction, as indicated by solid line,when the light is focused on the optical disk using an optical pickup200 prepared in this Example. In addition, FIG. 11( a) and FIG. 11( b)show changes in wavefront aberration λrms on an image surface withrespect to an amount of shift of the objective lens unit 213 in theoptical axis direction, as indicated by broken line, when the light isfocused on the optical disk using a conventional optical pickup preparedfor comparison. Note that, FIG. 11( a) is the result when the light wasfocused on the first optical disk, and FIG. 11( b) is the result whenthe light was focused on the second optical disk.

The conventional optical pickup for comparison was prepared so thatwavefront aberration is optimized when both the first and second lightbeams are incident on the objective lens unit as parallel rays, and whenthe second order component of the diffracted light is used for the firstlight beam and the first order component of the diffracted light is usedfor the second light beam.

As can be seen from FIG. 11( a), the optical pickup 200 can reduce thewavefront aberration for the first optical disk more effectively thanthe conventional optical pickup, thus forming a desirable focused lightspot.

As can be seen from FIG. 11( b), if the amount of shift of the objectivelens unit 213 is not more than 0.15 mm, the optical pickup 200 canreduce the wavefront aberration that is caused on the second opticaldisk by the shifting of the objective lens unit 213 when the secondlight beam L2 is incident on the objective lens unit 213 as a divergingray, more effectively than the conventional optical pickup.

That is, the optical pickup 200 can reduce wavefront aberration for thefirst and second optical disks more effectively than the conventionaloptical pickup.

Moreover, FIG. 12 shows changes in wavefront aberration λrms withrespect to changes in wavelength of the first light beam, as indicatedby solid line, when the light was focused on the first optical diskusing the optical pickup 200 prepared in this Example. In addition, FIG.12 shows changes in wavefront aberration λrms with respect to changes inwavelength of the first light beam L1 using a comparative optical pickupspecifically designated for the first optical disk.

The comparative optical pickup designated for the first optical disk isrealized by an objective lens unit which is solely composed of theobjective lens 212 (objective lens designated for the first light beam)used for the optical pickup 200 of this Example.

As can be seen from FIG. 12, the optical pickup 200 has a broader rangeof available wavelengths than the optical pickup designated for thefirst optical disk. This is because the diffraction optical element 211of the optical pickup 200 is realized by the converging diffractiongrating and the planoconcave lens. This makes it possible to improvewavelength dependency characteristic, as compared with the case of usingthe objective lens specifically designated for the first light beam.Therefore, the optical pickup 200 can form a desirable focused lightspot even in the presence of wavelength fluctuations caused by modehopping or the like.

EXAMPLE 4

In this example, as shown in FIG. 13( a) and FIG. 13( b), an objectivelens unit 213 is prepared so that a first light beam L1 (λ=405 nm)enters the objective lens unit 213 as a converging ray (i.e. at aconverging or diverging angle θ of −1.5°) and the objective lens unit213 uses the second order component of the light diffracted by adiffraction optical element 211, and that a second light beam L2 (λ=650nm) enters the objective lens unit 213 as a parallel ray (i.e., at aconverging or diverging angle θ of 0°) and the objective lens unit 213uses the first order component of the light diffracted by thediffraction optical element 211. An aspherical lens is adopted for anobjective lens 212. The diffraction optical element 211 has a concaveface that is also aspherical.

When the first light beam L1 enters the diffraction optical element 211as a converging ray, the light diffracted in the second orderdiffraction direction (in the converging direction with respect to theoptical axis) by a diffraction grating 211 a of the diffraction opticalelement 211 refracts in the diverging direction on a planoconcave lens211 b of the diffraction optical element 211 and enters the objectivelens 212 as a parallel ray. The diffracted light is then focused by theobjective lens 212, passes through a light transmissive layer (thicknessof 0.1 mm), and forms a small light spot on an information recordingface, thereby obtaining a desirable light focusing characteristic.

When the second light beam L2 enters the diffraction optical element 211as a parallel ray, the light diffracted in the first order diffractiondirection (in the converging direction with respect to the optical axis)by the diffraction grating 211 a of the diffraction optical element 211refracts in the diverging direction on the planoconcave lens 211 b ofthe diffraction optical element 211 and enters the objective lens 212 asa diverging ray. The diffracted light is then focused by the objectivelens 212, passes through a light transmissive layer (thickness of 0.6mm), and forms a small light spot on the information recording face,thereby obtaining a desirable light focusing characteristic.

By thus causing the first light L1 beam and the second light beam L2 toenter the diffraction optical element 211 as a converging ray and aparallel ray, respectively, the diffraction grating 211 a and theplanoconcave lens 211 b can be designed to have a pitch and a concaveface that satisfy a converging or diverging angle θ of 0° for the secondorder component of the diffracted light for the first light beam L1entering the objective lens 212, and a converging or diverging angle θof 2° to 3° for the first order component of the diffracted light forthe second light beam L2 entering the objective lens 212 (i.e., adiffraction angle difference of 2° to 3° is achieved between the secondorder component of the diffracted light for the first light beam L1 andthe first order component of the diffracted light for the second lightbeam L2).

Table 12 through Table 15 below show data concerning the respectivefaces of the objective lens 212 and diffraction optical element 211 sodesigned. Note that, the same symbols, numbers, and the like are used inTable 12 through Table 15 as in Tables 8 through Table 11.

TABLE 12 NEXT-GENERATION HIGH-DENSITY OPTICAL DISK (FIRST OPTICAL DISK)FACE CURVATURE FACE NUMBER RADIUS PITCH MATERIAL VIRTUAL LIGHT 0INFINITY −50 SOURCE DIFFRACTION 1 INFINITY 0.5 BK7_SCHOTT OPTICALELEMENT 2 3.620 0.5 OBJECTIVE LENS 3 1.465 2.5 LAH67_OHARA 4 20.516 0.1DISK 5 INFINITY 0.1 PC 6 INFINITY 0.23

TABLE 13 DVD (SECOND OPTICAL DISK) FACE CURVATURE FACE NUMBER RADIUSPITCH MATERIAL VIRTUAL LIGHT 0 INFINITY INFINITY SOURCE DIFFRACTION 1INFINITY 0.5 BK7_SCHOTT OPTICAL ELEMENT 2 3.620 0.5 OBJECTIVE LENS 31.465 2.5 LAH67_0HARA 4 20.516 0.1 DISK 5 INFINITY 0.6 PC 6 INFINITY0.10

TABLE 14 FACE NUMBER K A B C D E ASPHERICAL 2 2.68E+01 −4.63E−03 2.31E−03 −8.22E−04  1.22E−05 0.00E+00 FACTOR 4 −6.86E−01  1.33E−023.12E−03 2.57E−05 5.16E−04 −6.70E−05  5 7.22E+02 4.51E−01 −1.23E+00 1.51E+00 −9.37E−01  4.95E−01

TABLE 15 FACE NUMBER DF1 DF2 DF3 DF4 DF5 PHASE 1 −5.30E−03 −4.02E−04−1.17E−05 0.00E+00 0.00E+00 DIFFERENCE FACTOR

FIG. 14( a) and FIG. 14( b) show changes in wavefront aberration λrms onan image surface with respect to an amount of shift of the objectivelens unit 213 in the optical axis direction, as indicated by solid line,when the light is focused on the optical disk using an optical pickup200 prepared in this Example. In addition, FIG. 14( a) and FIG. 14( b)show changes in wavefront aberration λrms on an image surface withrespect to an amount of shift of the objective lens unit 20 in theoptical axis direction, as indicated by broken line, when the light isfocused on the optical disk using a conventional optical pickup preparedfor comparison. Note that, FIG. 14( a) is the result when the light wasfocused on the first optical disk, and FIG. 14( b) is the result whenthe light was focused on the second optical disk.

As with Example 3 above, the conventional optical pickup for comparisonis prepared so that wavefront aberration is optimized when both thefirst and second light beams are incident on the objective lens unit asparallel rays, and when the second order component of the diffractedlight is used for the first light beam and the first order component ofthe diffracted light is used for the second light beam.

As can be seen from FIG. 14( a), if the amount of shift of the objectivelens unit 213 is not more than 0.12 mm, the optical pickup 200 canreduce wavefront aberration caused on the first optical disk by theshifting of the objective lens unit 213 when the first light beam L1 isincident on the objective lens unit 213 as a converging ray, moreeffectively than the conventional optical pickup.

As can be seen from FIG. 14( b), the optical pickup 200 can reduce thewavefront aberration for the second optical disk more effectively thanthe conventional optical pickup, thus forming a desirable focused lightspot.

That is, with the optical pickup 2, wavefront aberration for the firstand second optical disks can be reduced more effectively than theconventional optical pickup.

EXAMPLE 5

In this example, as shown in FIG. 15( a) and FIG. 15( b), an objectivelens unit 213 is prepared so that a first light beam L1 (λ=405 nm)enters the objective lens unit 213 as a converging ray (i.e. at aconverging or diverging angle θ of −0.8°) and the objective lens unit213 uses the second order component of the light diffracted by adiffraction optical element 211, and that a second light beam L2 (λ=650nm) enters the objective lens unit 213 as a diverging ray (i.e., at aconverging or diverging angle θ of 0.8°) and the objective lens unit 213uses the first order component of the light diffracted by thediffraction optical element 211. An aspherical lens is adopted for anobjective lens 212. The diffraction optical element 211 has a concaveface that is also aspherical.

When the first light beam L1 enters the diffraction optical element 211as a converging ray, the light diffracted in the second orderdiffraction direction (in the converging direction with respect to theoptical axis) by a diffraction grating 211 a of the diffraction opticalelement 211 refracts in the diverging direction on a planoconcave lens211 b of the diffraction optical element 211 and enters the objectivelens 212 as a parallel ray. The diffracted light is then focused by theobjective lens 212, passes through a light transmissive layer (thicknessof 0.1 mm), and forms a small light spot on an information recordingface, thereby obtaining a desirable light focusing characteristic.

When the second light beam L2 enters the diffraction optical element 211as a diverging ray, the light diffracted in the first order diffractiondirection (in the converging direction with respect to the optical axis)by the diffraction grating 211 a of the diffraction optical element 211refracts in the diverging direction by the planoconcave lens 211 b ofthe diffraction optical element 211 and enters the objective lens 212 asa diverging ray. The diffracted light is then focused by the objectivelens 212, passes through a light transmissive layer (thickness of 0.6mm), and forms a small light spot on the information recording face,thereby obtaining a desirable light focusing characteristic.

By thus causing the first light beam L1 and the second light beam L2 toenter the diffraction optical element 211 as a converging ray and adiverging ray, respectively, the diffraction grating 211 a and theplanoconcave lens 211 b can be designed to have a pitch and a concaveface that satisfy a converging or diverging angle θ of 0° for the secondorder component of the diffracted light for the first light beam L1entering the objective lens 212, and a converging or diverging angle of2° to 3° for the first order component of the diffracted light for thesecond light beam L2 entering the objective lens 212 (i.e., adiffraction angle difference of 2° to 3° is achieved between the secondorder component of the diffracted light for the first light beam L1 andthe first order component of the diffracted light for the second lightbeam L2).

Table 16 through Table 19 below show data concerning the respectivefaces of the objective lens 212 and diffraction optical element 211 sodesigned. Note that, the same symbols, numbers, and the like are used inTable 16 through Table 19 as in Tables 8 through Table 11.

TABLE 16 NEXT-GENERATION HIGH-DENSITY OPTICAL DISK (FIRST OPTICAL DISK)FACE CURVATURE FACE NUMBER RADIUS PITCH MATERIAL VIRTUAL LIGHT 0INFINITY −100.000 SOURCE DIFFRACTION 1 INFINITY 0.5 BK7_SCHOTT OPTICALELEMENT 2 12.520 0.5 OBJECTIVE LENS 3 1.465 2.5 LAH67_OHARA 4 20.516 0.1DISK 5 INFINITY 0.1 PC 6 INFINITY 0.23

TABLE 17 DVD (SECOND OPTICAL DISK) FACE CURVATURE FACE NUMBER RADIUSPITCH MATERIAL VIRTUAL LIGHT 0 INFINITY INFINITY SOURCE DIFFRACTION 1INFINITY 0.5 BK7_SCHOTT OPTICAL ELEMENT 2 12.520 0.5 OBJECTIVE LENS 31.465 2.5 LAH67_OHARA 4 20.516 0.1 DISK 5 INFINITY 0.6 PC 6 INFINITY0.10

TABLE 18 FACE NUMBER K A B C D E ASPHERICAL 2 0.00E+00 −5.6E−03 −1.1E−03 −3.4E−03  1.4E−03 0.0E+00 FACTOR 4 −6.9E−01 1.3E−02  3.1E−032.6E−05 5.2E−04 −6.7E−05  5  7.2E+02 4.5E−01 −1.2E+00 1.5E+00 −9.4E−01 5.0E−01

TABLE 19 FACE NUMBER DF1 DF2 DF3 DF4 DF5 PHASE 1 −9.5E−03 2.5E−032.6E−04 1.7E−03 −6.4E−04 DIFFERENCE FACTOR

FIG. 16( a) and FIG. 16( b) show changes in wavefront aberration λrms onan image surface with respect to an amount of shift of the objectivelens unit 213 in the optical axis direction, as indicated by solid line,when the light is focused on the optical disk using an optical pickup200 prepared in this Example. In addition, FIG. 16( a) and FIG. 16( b)show changes in wavefront aberration λrms on an image surface withrespect to an amount of shift of the objective lens unit 213 in theoptical axis direction, as indicated by broken line, when the light isfocused on the optical disk using a conventional optical pickup preparedfor comparison. Note that, FIG. 16( a) is the result when the light wasfocused on the first optical disk, and FIG. 16( b) is the result whenthe light was focused on the second optical disk.

As with Example 4 above, the conventional optical pickup for comparisonis prepared so that wavefront aberration is optimized when both thefirst and second light beams are incident on the objective lens unit asparallel rays, and when the second order component of the diffractedlight is used for the first light beam and the first order component ofthe diffracted light is used for the second light beam.

As can be seen from FIG. 16( a) and FIG. 16( b), the optical pickup 200can reduce the wavefront aberration for the first optical disk moreeffectively than the conventional optical pickup, thus forming anexcellent focus spot.

Further, in the optical pickup 200, the adverse effect of shifting ofthe objective lens unit 213, which is caused when the first and secondlight beams are incident on the objective lens unit 213 as a convergingray and a diverging ray, respectively, can be reduced more effectivelythan the conventional optical pickup when the amount of shift of theobjective lens unit 213 causing wavefront aberration is not more than0.23 mm for the first optical disk and when the amount of shift of theobjective lens unit 213 causing wavefront aberration is not more than0.3 mm for the second optical disk.

By thus causing the first light beam and the second light beam to enterthe objective lens unit 213 as a converging ray and a diverging ray,respectively, the wavefront aberration can be reduced over a relativelywide shift range even when the objective lens unit 213 shifts in theoptical axis direction of the first and second light beams.

In Examples 3 through 5, the diffraction optical element 211 has anaspherical concave face. However, the same effect can also be obtainedwith a spherical concave face. Fabrication of the diffraction opticalelement 211 is easier when the concave face is spherical than it is whenaspherical. Thus, with a spherical concave face, the diffraction opticalelement 211 can be provided inexpensively, reducing the cost therefor.

In the optical pickup 200, the first and second light beams L1 and L2enter the diffraction optical element 211 as light beams with differentdegrees of convergence or divergence, which makes it easier to provide alarge angle difference. This enables the required diffracting andrefracting characteristics for the diffraction optical element 211 to beset more freely, allowing for more flexible design for the diffractionoptical element 211. As a result, using the diffraction optical element211, which is easy to fabricate, it is possible to realize the opticalpickup 200 which can sufficiently reduce wavefront aberration in focuseddiffracted light.

Note that, when the first and second light beams L1 and L2 enter thediffraction optical element 211 as light beams with different degrees ofconvergence or divergence, the incident ray of one of the first andsecond light beams may be a converging ray while the other is adiverging ray, as described in Example 3 through Example 5.Alternatively, the incident ray of one of the first and second lightbeams may be a parallel ray while the other is a converging ray or adiverging ray. Further, the incident rays of both the first light beamand the second light beams may be converging rays or diverging rays withdifferent degrees of convergence or divergence.

When the respective light beams of blue and red are incident on thediffraction optical element 211 as parallel rays, the angle differencebetween the diffraction angle for the blue light and the diffractionangle for the red light, which is required to compensate for thespherical aberration caused by the large difference in thickness of thelight transmissive layers, must be increased to about 2° to 3° in orderto provide compatibility for the next-generation high-density opticaldisk and the conventional DVD. The angle difference is related to thepitch of the diffraction grating 211 a of the diffraction opticalelement 211, as shown by the graph of FIG. 42. It can be seen from FIG.42 that the pitch of the diffraction grating needs to be as narrow as3.5 μm to 4.5 μm in order to achieve the angle difference of about 2° to3°.

Further, since the objective lens (infinite objective lens) is generallyoptimized for the blue light approaching from a point of infinity, theemergent ray from the diffraction optical element needs to be a parallelray. That is, a ray of blue light that is bent on the diffracting faceof the diffraction optical element needs to be refracted to a parallelray on entering the refracting face (face of the diffraction opticalelement on the side of the objective lens). This is also effective inpreventing aberration caused by misalignment of the diffraction opticalelement with the objective lens.

FIG. 43 represents a relationship between pitch of the diffractiongrating and curvature of the refracting face of the diffraction opticalelement, when a parallel ray of blue light incident on the diffractionoptical element emerges from the diffraction optical element also as aparallel ray. Note that, the relationship represented in FIG. 43 isbased on a diffraction optical element in an optical pickup using anobjective lens with an effective radius of 2 mm. The refracting face ofthe diffraction optical element is spherical. It can be seen from FIG.43 that the curvature radius of the refracting face of the diffractionoptical element needs to be no greater than 2.2 mm in order to confinethe pitch of the diffraction grating from 3.5 μm to 4.5 μm.

However, given the fact that the effective radius of a common objectivelens is 2 mm, and that the effective diameter of the diffraction opticalelement is also 2 mm, the refracting face with a curvature radius of nogreater than 2.2 mm is substantially hemispherical, which is impossibleto fabricate or practically useless. The refracting face may be madeaspherical, but in this case the exceedingly small curvature makesfabrication of the diffraction optical element difficult. Even if it ispossible to fabricate, the on-axis focusing characteristic isundesirably increased to 0.018λ(rms) for all of the optical disks.

As described, these and other problems can be solved by the opticalpickup 200 of the present embodiment. That is, the optical pickup 200 isable to record or reproduce information with respect to recording mediarespectively having light transmissive layers of different thicknessesand respectively using different optimum wavelengths of light forreproducing. In addition, the optical pickup 200 is easy to fabricate,and can sufficiently reduce aberration in the focused light beam.

The optical pickup of the present embodiment as shown in FIG. 9 may bemodified to provide an optical pickup as illustrated in FIG. 45. Theoptical pickup shown in FIG. 45 has the same structure as the opticalpickup of FIG. 9, except that the ¼ wavelength plate 8 and the sphericalaberration compensation system 6 are switched in position in the opticalpickup 200.

Third Embodiment

FIG. 17 illustrates a schematic structure of an optical pickup 300 ofthe present embodiment. The description of the present embodiment willbe given based on the optical pickup 300 that is compatible with anext-generation high-density optical disk (first optical disk 14 a,first recording medium), a conventional DVD (second optical disk 14 b,second recording medium), and a conventional CD (third optical disk 14c, third recording medium).

The first optical disk 14 a uses blue light (first light beam) of ashort wavelength in the vicinity of 405 nm (first wavelength λ1), andhas a light transmissive layer with a thickness t1=0.1 mm. The secondoptical disk 14 b uses red light (second light beam) of a longwavelength in the vicinity of 650 nm (second wavelength λ2), and has alight transmissive layer with a thickness t2=0.6 mm. The third opticaldisk 14 c uses infrared light (third light beam) of a long wavelength inthe vicinity of 780 nm (third wavelength λ3), and has a lighttransmissive layer with a thickness t3 =1.2 mm.

The optical pickup 300 includes a semiconductor laser 1 a that emits thefirst light beam of the first wavelength λ1, a semiconductor laser 1 bthat emits the second light beam of the second wavelength λ2 longer thanλ1, and a semiconductor laser 1 c that emits the third light beam of thethird wavelength λ3 longer than λ2. The semiconductor laser 1 a,semiconductor laser 1 b, and semiconductor laser 1 c (light source) areswitched (turned on) according to the type of target optical disk.

The optical pickup 300 further includes collimator lenses 2 a and 2 b,shaping optical systems 3 a and 3 b, and beam splitters 4 a and 4 b.Through the collimator lenses 2 a and 2 b, the first and second lightbeams respectively emerged from the semiconductor lasers 1 a and 1 bbecome substantially parallel rays. The shaping optical systems 3 a and3 b, such as a shaping prism, shape an ellipsoidal intensitydistribution of the first and second light beams into a substantiallycircular intensity distribution. The beam splitters 4 a and 4 b pass thefirst and second light beams from the shaping optical systems 3 a and 3b.

The shaping optical systems 3 a and 3 b are realized by a known opticalsystem, which may be a single triangular prism, a combined triangularprism, or two discrete triangular prisms. It should be noted that theshaping optical systems 3 a and 3 b are optional in the optical pickup300.

Further, the optical pickup 300 includes a compensating lens 2 c throughwhich the third light beam emerged from the semiconductor laser 1 cbecomes a predetermined diverging ray, and a beam splitter 4 c thatpasses the third light emerged from the compensating lens 2 c. Here, thecompensating lens 2 c is an aspherical lens inserted to reduce theadverse effect of radial shifting of the objective lens unit 313.

The semiconductor laser 1 a, the collimator lens 2 a, the shapingoptical system 3 a, and the beam splitter 4 a make up a first opticalsystem 16 a. The semiconductor laser 1 b, the collimator lens 2 b, theshaping optical system 3 b, and the beam splitter 4 b make up a secondoptical system 16 b. The semiconductor laser 1 c, the compensating lens2 c, the beam splitter 4 c make up a third optical system 16 c.

The first and second light beams respectively emerged from the first andsecond optical systems 16 a and 16 b enter a dichroic mirror 5 wheretheir optical axes merge, and through a spherical aberrationcompensation system 6 they change their degrees of convergence and/ordivergence according to the type of light beam (first light beam orsecond light beam). Thereafter, the first and second light beams enter adichroic mirror 7 where their optical axes merge with optical axis ofthe third light beam emerged from the third optical system 16 c. Leavingthe dichroic mirror 7, the first, second, and third light beams travelthrough a common optical system.

Here, the spherical aberration compensation system 6 serves as a beamexpander to compensate for the spherical aberration caused by thethickness difference or other properties of the light transmissivelayers of the first and second optical disks 14 a and 14 b, as well aslight beam control means for changing the degree of convergence and/ordivergence of the first and second light beams, as described above. Notethat, the compensating lens 2 c serves as light beam control means forchanging the degree of convergence and/or divergence of the third lightbeam.

In a configuration where the optical pickup 300 does not include theshaping optical systems 3 a and 3 b, the collimator lens 12 a and 12 bmay be used instead of the spherical aberration compensation system 6 tochange the degree of convergence and/or divergence of the first andsecond light beams. In addition, elements other than the sphericalaberration compensation system 6 or the collimator lenses 12 a and 12 bmay be used to change the degree of convergence and/or divergence of thefirst and second light beams.

In the common optical system, the first, second, and third light beamspass through a ¼-wavelength plate 8, and are reflected by a mirror 9into an objective lens unit 313.

Entering the objective lens unit 313, the first, second, third lightbeams travel through a wavelength-selective aperture filter 310, adiffraction optical element 311, and an objective lens 312 in thisorder, and form a small light spot on an information recording face ofthe first optical disk 14 a, second optical disk 14 b, or third opticaldisk 14 c.

Further, the wavelength-selective aperture filter 310 controls apertureso that a numerical aperture NA1 (0.85 to be specific), a numericalaperture NA2 (0.6 to be specific), and a numerical aperture NA3 (0.45 tobe specific) are obtained for the first, second, and third light beamsof the first, second, and third wavelengths λ1, λ2, and λ3,respectively. Note that, the wavelength-selective aperture filter 310,which is provided between the mirror 9 and the diffraction opticalelement 311 in this embodiment, may be disposed in any other position,provided that the wavelength-selective aperture filter 310 is operativeas an integral unit with the diffraction optical element 311 and theobjective lens 312, and that the wavelength-selective aperture filter310 is disposed between the objective lens 312 and light source.Further, the wavelength-selective aperture filter 310 may be realized byany other member, provided that it serves to control aperture.

The wavelength-selective aperture filter 310, the diffraction opticalelement 311, and the objective lens 312 are integrally provided as theobjective lens unit 313, and can move relative to other optical systemsin the optical pickup 300 in the direction of optical axis, i.e. in theZ direction as indicated by arrow in FIG. 17. This allows the light spotof focused light to desirably follow the oscillations of the informationrecording faces of the first, second, third optical disks 14 a, 14 b,and 14 c, or rotation eccentricity of information tracks of the first,second, third optical disks 14 a, 14 b, and 14 c.

In addition to the foregoing light projecting optical system, theoptical pickup 300 further includes reproduced signal detecting opticalsystems 15 a, 15 b, and 15 c. The reproduced signal detecting opticalsystems 15 a, 15 b, and 15 c are realized by a known optical system, andserve to reproduce a light spot control signal for auto focusing ortracking, or an information signal recorded in the optical disk.

The objective lens unit 313 is an assembly integrally composed of: theobjective lens 312 that focuses the first, second, and third light beamsonto the information recording faces of the first, second, and thirdoptical disk 14 a, 14 b, and 14 c, respectively; the diffraction opticalelement 311 having a planoconcave lens 311 b, which is a translucentdiverging lens, and a converging diffraction grating 311 a formed on asurface of the planoconcave lens; and the wavelength-selective aperturefilter 310.

The diffraction optical element 311 is made with the convergingdiffraction grating 311 a and the diverging planoconcave lens 311 b, soas to suppress the wavefront aberration caused by wavelengthfluctuations and to obtain desirable light focusing characteristicsagainst wavelength fluctuations. Note that, an amount of aberration isreferred to herein as wavefront aberration.

The diffraction optical element 311 is made by forming the diffractiongrating 311 a on a plane face of the planoconcave lens 311 b, so thatthe number of components can be reduced. Alternatively, the diffractionoptical element 311 may be composed of two optical elements made up of adiffraction element and a lens. In either case, the face of thediffraction optical element 311 with the diffracting action and the facewith the refracting action are referred to as a diffracting face and arefracting face, respectively.

Note that, in the present optical pickup 300, the diffraction grating311 a of the diffraction optical element 311 is provided on the side ofthe wavelength-selective aperture filter 310 (on the side of lightsource); however, provision of the diffraction grating 311 a on the sideof the objective lens 312 can also bring about the same or similareffect.

In addition, the objective lens unit 313 may be realized by adiffraction optical element 311 including the diffraction grating on theconcave face (refracting face) of the translucent lens on the other sideof the plane face. In this case, alignment of the concave face with thediffraction grating becomes easier.

The planoconcave lens 311 b of the diffraction optical element 31 1 ismade of glass or plastic, for example. The diffraction grating 311 a ofthe diffraction optical element 311 has concentrically grooved ringsaround the optical axis, or raised orbicular bands formed byphotolithography around the optical axis on the plane face of theplanoconcave lens 311 b. Alternatively, the diffraction grating of thediffraction optical element 311 may be concentrically formed around theoptical axis by glass molding or resin molding.

Preferably, the diffraction grating 311 a is formed so that the crosssection that cuts across the plane including the optical axis is blazed,i.e. serrated. The diffraction grating with the serrated cross sectionis advantageous over other types of diffraction gratings because itoffers higher diffraction efficiency. Further, the diffraction grating311 a may be formed so that the cross section that cuts across the planeincluding the optical axis is stepped. The diffraction grating with thestepped cross section is advantageous over other types of diffractiongratings because it offers higher diffraction efficiency next to thediffraction grating with the serrated cross section.

Here, the objective lens 312 is designed so as to compensate for theaberration caused on the information recording face of the first opticaldisk 14 a (next-generation high-density optical disk: using thewavelength of 405 nm; light transmissive layer of 0.1 mm) using thefirst light beam (blue light: λ=405 nm).

The degrees of convergence or divergence for the first, second, andthird light beams after they have passed through the diffraction opticalelement 311 are respectively denoted as Φoutb, Φoutr, and ΦoutIr in thefollowing Equation (15).Φ_(outb)=Φ_(inb)+Φ_(HOEb)+Φ_(Lb)Φ_(outr)=Φ_(inr)+Φ_(HOEr)+Φ_(Lr)Φ_(outIr)=Φ_(inIr)+Φ_(HOEIr)+Φ_(Lir)   (15),where Φinb, Φinr, and ΦinIr are respectively the degrees of convergenceor divergence of the first, second, and third light beams entering thediffraction optical element 311, ΦHOEb, ΦHOEb, and ΦHOEIr arerespectively the powers of the diffracting face of the diffractionoptical element 311 for the first, second, and third light beams, andΦLb, ΦLr, and ΦLIr are respectively the powers of the refracting face ofthe diffraction optical element 311 for the first, second, and thirdlight beams.

Referring to FIG. 18, the following defines the degree of convergence ordivergence and the power of the respective face. FIG. 18 is a crosssectional view as viewed in a direction orthogonal to the optical axis,showing a state of light rays passing through the diffraction opticalelement 311 made of a material with a refractive index n. It can be seenthat rays from an object point O are diffracted on the diffracting faceT and refracted on the refracting face S.

The degree of convergence or divergence Φin of an incident ray on thediffraction optical element 311 (degree of incident convergence orincident divergence) is an inverse of distance s between the objectpoint O and point A where the optical axis intersects with thediffracting face T. That is, the degree of convergence or divergence ofthe light that enters the diffracting face T at angle u locating a pointof incidence distanced by h from point A where the optical axisintersects with the diffracting face T can be expressed by the followingEquation (16).φ×Φ_(in)=tan u/h   (16)

Further, the degree of convergence or divergence Φout of an emergent rayfrom the diffraction optical element 311 (degree of emergent convergenceor emergent divergence) is an inverse of distance s′ from the refractingface S to an object point O′ where the emergent ray intersects with theoptical axis after passage through the diffraction optical element 311.That is, the degree of convergence or divergence of the light thatemerges from the refracting face S at angle u′ locating a point ofemergence distanced by h′ from peak B of the refracting face S can beexpressed by the following Equation (17).φ×Φ_(out)=tan u′/h′  (17)

Note that, in this specification, rays are diverging rays when thedegree of convergence or divergence has a negative value, and areconverging rays when the degree of convergence or divergence has apositive value. Further, the degree of convergence or divergence may beexpressed as φ×Φin and φ×Φout by multiplying Φin and Φout with φ, whichis the effective diameter of the objective lens 312 for the first lightbeam.

The powers ΦLb, ΦLr, ΦLIr of the refracting face S can be expressed byEquation (18) below. Note that, in this specification, powers arediverging when they have a negative value, and are converging when theyhave a positive value.Φ_(bL)=(1−n _(b))/RΦ_(Lr)=(1−n _(r))/RΦ_(LIr)=(1−n _(Ir))/R   (18)

Here, nb, nr, nIr are respectively the refractive indices of thediffraction optical element 311 for the first, second, and third lightbeams, and R is the curvature radius of the refracting face S. Further,the power ΦHOE of the diffracting face T can be obtained from an opticalpath difference function, which represents a shape of the diffractingface T. Note that, the power of the refracting face S and the power ΦHOEof the diffracting face T are equivalent in terms of their ability torefract incident light.

In the case where no aberration is caused on the first optical disk 14 aby the first light beam entering the objective lens 312 as a parallelray, the degrees of convergence and/or divergence for the second andthird light beams emerging from the diffraction optical element 311(degrees of convergence and/or divergence of the incident ray on theobjective lens) necessary to compensate for the spherical aberrationcaused by the difference in thickness of the light transmissive layersof the second and third optical disks 14 b and 14 c should generally beconfined within the ranges defined by Inequalities (19) below.−0.16≦φ×Φ_(outr)≦−0.05−0.26≦φ×Φ_(outIr)≦−0.15   (19),where φ is the effective diameter of the objective lens 312 for thefirst light beam.

The optical pickup 300 is designed to satisfy the following Inequalities(20) using the diffraction optical element 311.|Φ_(outr)|>|Φ_(inr)|, and |Φ_(outIr)|>|Φ_(inIr)|  (20)

By satisfying these inequalities, the light beams can enter theobjective lens 312 with sufficient degrees of divergence so as tocompensate for the spherical aberration caused by the difference inthickness of the light transmissive layers, even when the second andthird light beams are incident on the objective lens unit 313 withdegrees of divergence of relatively small absolute values. By thusenabling the second and third light beams to be incident on theobjective lens unit 313 with degrees of divergence of relatively smallabsolute values, it is possible to reduce the adverse effect of radialshifting of the objective lens unit 313 (in the substantially orthogonaldirection to the optical axes of the incident first, second, and thirdlight beams; in a direction of arrow Z in FIG. 17) caused by tracking orother operations.

The values that satisfy Inequalities (15) are determined by thefollowing: (a) the optical path difference function which represents ashape of the diffracting face; (b) the diffraction orders of the first,second, and third light beams used; (c) the degrees of convergenceand/or divergence for the first, second, and third light beams enteringthe diffraction optical element 311; (d) the curvature radius of therefracting face; and (e) the refractive index of the diffraction opticalelement 311.

In the optical pickup 300, Φoutb is preferably 0. This allows the firstlight beam of a short wavelength, which requires the most accuracy forthe wavefront aberration, to be incident on the objective lens 312 as aparallel ray. As a result, when using the first light beam, it ispossible to suppress the aberration caused by misalignment of thediffraction optical element 311 with the objective lens 312.

The following explains how diffraction orders of the light beamsdiffracted on the diffracting face of the diffraction optical element311 are used. The diffraction efficiency of the diffraction grating withthe blazed cross section can be obtained by the following Equation (21).

$\begin{matrix}{\eta_{m} = {{\frac{1}{T}{\int_{0}^{T}{{A(x)}\exp\left\{ {i\;\phi\;(x)} \right\}{\exp\left( {{- i}\;\frac{{2\pi\;{mx}}\;}{T}} \right)}{\mathbb{d}x}}}}}^{2}} & (21)\end{matrix}$

In this equation, m is the diffraction order, A(x) is the transmittedamplitude distribution, φ(x) is the phase distribution, and T is theperiod length in the x-axis direction. Note that, calculations belowhave been normalized with A(x)=1. FIG. 19 illustrates the result ofcalculation using Equation (5) when diffraction efficiencies werespecifically calculated using a diffraction grating made out of a PC(polycarbonate) base.

In FIG. 19, B0, B1, B2 are diffraction efficiencies for the zerothorder, first order, and second order components of the diffracted lightfor the first light beam, respectively, R0, R1, R2 are diffractionefficiencies for the zeroth order, first order, and second ordercomponents of the diffracted light for the second light beam,respectively, and Ir0, Ir1, and Ir2 are diffraction efficiencies for thezeroth order, first order, and second order components of the diffractedlight for the third light beam, respectively.

The optical pickup 300 is adapted to use the second order component ofthe diffracted light for the first light beam and the first ordercomponent of the diffracted light for the second and third light beams.This can increase the efficiency of using each of the first, second, andthird light beams.

Specifically, when PC is used as the material of the diffraction opticalelement, as shown in FIG. 19, the diffraction grating approximately 1.3μm in depth allows for approximately 100% efficiency for the first lightbeam and 90% or greater efficiency for the second and third light beams.This makes it possible to readily realize an optical pickup thatperforms information recording and erasing that requires a high powerlight beam. In addition, the power of the light source can be madesmaller, reducing power consumption. Further, since there is nounnecessary light other than the diffracted light used, it is possibleto prevent stray light from entering a detector such as the reproducedsignal detecting optical systems 15 a during reproduction, therebysuppressing deterioration of reproduced signals.

FIG. 20 illustrates a relationship between degrees of convergence and/ordivergence for the first, second, and third light beams incident on thediffraction optical element 311. FIG. 20 shows changes in degrees ofincident convergence and/or incident divergence for the second and thirdlight beams (φ×Φinr and φ×ΦinIr) with changes in degree of incidentconvergence or incident divergence for the first light beam (φ×Φinb).

Here, the first light beam (emergent ray from the diffraction opticalelement 311) is incident on the objective lens 312 as a parallel ray(φ×Φoutb=0).

The second light beam (emergent ray from the diffraction optical element311) is incident on the objective lens 312 as a diverging ray(φ×Φoutr=−0.1), and the third light beam (emergent ray from thediffraction optical element 311) is incident on the objective lens 312as a diverging ray (φ×ΦoutIr=−0.2). These degrees of divergence areapproximate mean values of Inequalities (19), which define the ranges ofdivergence degree generally required to compensate for the sphericalaberration caused by the difference in thickness of the lighttransmissive layers of the optical disks 14 b and 14 c from that of thefirst optical disk 14 a, when the second and third light beams arefocused on the second and third optical disks 14 b and 14 c. The degreesof divergence given above are also values that yield the bestcompensation result.

Further, the diffracting face of the diffraction optical element 311 isdesigned such that the second order component of the diffracted light isused for the first light beam, and the first order component of thediffracted light is used for the second and third light beams. Thediffraction optical element 311 is made with a refracting face with acurvature radius of 5 mm, and the material of the diffraction opticalelement 311 is PC. The degree of incident convergence or incidentdivergence for the first light beam is varied by changing the shape ofthe diffracting face of the diffraction optical element 311.

As can be seen from FIG. 20, the degree of incident convergence orincident divergence for the first light beam is directly proportional tothe degrees of incident convergence or incident divergence of the secondand third light beams. Further, with a negative value of the degree ofconvergence or divergence for the third light beam incident on thediffraction optical element 311, i.e. when the third light beam entersthe diffraction optical element 311 as a diverging ray, the degrees ofincident convergence or incident divergence for the first and secondlight beams can remain at relatively small absolute values. Further,when the degree of incident convergence or incident divergence for thefirst light beam is not less than 0, i.e., when the first light beamenters the diffraction optical element 311 as a parallel or a convergingray, the degrees of incident convergence or incident divergence for thesecond and third light beams have small absolute values.

It is therefore preferable that the first and second light beams enterthe diffraction optical element 311, for example, as a parallel ray anda diverging ray, respectively. By causing the first light beam, whichrequires the most accurate light focusing characteristic, to be incidenton the diffraction optical element 311 as a parallel ray, the degrees ofdivergence for the second and third light beams entering the diffractionoptical element 311 can remain relatively small. As a result, it ispossible to effectively suppress the impairment of light focusingcharacteristic due to radial shifting of the objective lens unit 313.

Alternatively, it is preferable that the first light beam is incident onthe diffraction optical element 311 as a converging ray, and the secondlight beam is incident on the diffraction optical element 311 as aconverging, parallel, or diverging ray. In this way, the degrees ofconvergence or divergence for the first, second, and third light beamsentering the diffraction optical element 311 can remain at relativelysmall absolute values. As a result, it is possible to effectivelysuppress the impairment of light focusing characteristic due to radialshifting of the objective lens unit 313.

FIG. 21 verifies this in detail. FIG. 21 shows a relationship betweenwavefront aberration and degree of divergence for the first light beamentering the diffraction optical element 311, when the amount of radialshift of the objective lens unit 313 is 200 μm.

Referring to FIG. 20, in the optical pickup 300, when the degree ofconvergence or divergence for the first light beam entering thediffraction optical element 311 is in the range of Inequality (22)below, the degrees of convergence or divergence for the second and thirdlight beams entering the diffraction optical element 311 satisfyInequalities (23) and (24) below, respectively.

$\begin{matrix}{0 \leq {\phi \times \Phi_{inb}} \leq 0.11} & (22) \\{{- 0.048} \leq {\phi \times \Phi_{inr}} \leq 0.04} & (23) \\{{- 0.18} \leq {\phi \times \Phi_{{inI}\; r}} \leq {- 0.1}} & (24)\end{matrix}$

In this case, as shown in FIG. 21, it is possible to reduce wavefrontaberration to not more than 0.04 λrms for all of the first, second, andthird light beams.

Further, in this case, the aforementioned Inequalities (20) aresatisfied, as can be seen from FIG. 21. This can reduce the degrees ofconvergence or divergence for the second and third light beams enteringthe diffraction optical element 311, thereby reducing the adverse effectof radial shifting of the objective lens unit 313 caused by tracking orother operations.

Here, the respective powers Φb, Φr, and ΦIr of the diffraction opticalelement 311 for the first, second, and third light beams are defined byEquations (25) below. That is, the power of the diffraction opticalelement 311 is defined by a sum of the power of the diffracting face(ΦHOE) and the power of the refracting face (ΦL).Φ_(b)=Φ_(HOEb)+Φ_(Lb)Φ_(r)=Φ_(HOEr)+Φ_(Lr)Φ_(Ir)=Φ_(HOEIr)+Φ_(LIr)   (25)

From Equations (15) and (25), the following Equation (26) can beobtained.Φ_(b)=Φ_(outb)−Φ_(inb)Φ_(r)=Φ_(outr)−Φ_(inr)Φ_(Ir)=Φ_(outIr)−Φ_(inIr)   (26)

Therefore, in the optical pickup 300, from Equations (22), (23), and(24) defining ranges of Φin, and from Φoutb=0 defining a range of Φout,and from Inequalities (19), the respective powers of the diffractionoptical element 311 satisfy the following Inequalities (27).−0.11≦φ×Φ_(b)≦0−0.2≦φ×Φ_(r)≦−0.002−0.16≦φ×Φ_(Ir)≦0.03   (27)

According to this, it is possible to obtain desirable wavefrontaberration regardless of the presence or absence of radial shifting ofthe objective lens unit 313.

The respective powers of the diffracting face and refracting face of thediffraction optical element 311 can be suitably set in the foregoingpower ranges for the diffraction optical element 311. In the opticalpickup 300, the diffracting face has a positive power (convergingdiffracting face) and the refracting face has a negative power (concaveface). This makes it possible to reduce an increase of the wavefrontaberration caused when the wavelength of the light source shifts, aswill be described later.

Here, by setting the degree of convergence or divergence for the firstlight beam entering the diffraction optical element 311 so that φ×Φinb=0(causing the first light beam to be incident on the diffraction opticalelement 311 as a parallel ray), it is possible to reduce the wavefrontaberration caused by radial shifting of the objective lens unit 313,also for the first light beam which requires more accurate lightfocusing characteristic due to its short wavelength. When φ×Φoutb=0(causing the first light beam to be incident on the objective lens 312as a parallel ray), the power of the diffraction optical element 311 forthe first light beam is set so that Φb=0. Details of this will beexplained in Example 6.

As another example, when the degree of divergence for the first lightbeam entering the diffraction optical element 311 is φ×Φinb=0.06 (thefirst light beam enters the diffraction optical element 311 as aconverging ray), φ×Φinr≈0 (the incident ray is substantially parallel)and φ×ΦinIr≈−0.15 (the incident ray is diverging) (see FIG. 20). In thiscase, it is possible to reduce the wavefront aberration caused by radialshifting of the objective lens unit 313, also for the second light beam(see FIG. 21). Details of this will be explained in Example 7.

Further, by causing the first, second, and third light beams to enterthe diffraction optical element 311 with predetermined degrees ofconvergence and/or divergence as described above, the wavefrontaberration can be suppressed even when the objective lens unit 313shifts in the radial direction. However, the wavefront aberration can besuppressed more effectively when an aspherical lens is inserted betweenthe light source and the diffraction optical element 311, so as toreduce coma aberration caused by radial shifting of the objective lensunit 313, when a diverging ray is incident on the diffraction opticalelement 311.

EXAMPLE 6

Another Example of the present invention is described below withreference to FIGS. 22( a), (b), (c), FIGS. 23( a), (b), (c) and FIG. 24.

In this Example, as shown in FIG. 22( a), FIG. 22( b) and FIG. 22( c),the first light beam is incident on the diffraction optical element 311with a degree of convergence or divergence φ×Φinb=0, so that the adverseeffect of radial shifting of the objective lens unit 313 can beeliminated almost completely for the first light beam which requires themore accurate light focusing characteristic. Note that, this Exampleuses an objective lens 312 with an effective diameter φ=3 mm for thefirst light beam L1.

An optical pickup 3 of this example is fabricated so that the firstlight beam L1 is incident on the diffraction optical element 311 as aparallel ray given by φ×Φinb=0, while the second and third light beamsL2, L3 are incident on the diffraction optical element 311 as divergingrays, which are given by φ×Φinr=−0.048 and φ×ΦinIr=−0.18, respectively.The diffracting face of the diffraction optical element 311 is designedsuch that the second order component of the diffracted light is used forthe first light beam, and the first order component of the diffractedlight is used for the second and third light beams.

The diffraction optical element 311 is made with a concave face and adiffraction grating, and is disposed on the side of the light source forthe aspherical objective lens 312. The concave face is spherical and hasa curvature radius of 5 mm.

Here, the concave face is spherical because it is easier to fabricate.The concave face may be made aspherical to further improve shiftingcharacteristics of the objective lens unit 313 in the radial direction.

In the optical pickup 300 of this Example, as shown in FIG. 22( a), forthe first optical disk 14 a, the first light beam L1 is incident on thediffraction optical element 311 as a parallel ray given by φ×Φinb=0, andthe light beam diffracted in a second order diffraction direction(converging direction with respect to the optical axis) on thediffracting face is refracted in a diverging direction on the concaveface so that the light beam is incident on the objective lens 312 as aparallel ray and is focused on the first optical disk 14 a having a 0.1mm thick light transmissive layer. In this way, desirable light focusingcharacteristics are obtained.

For the second optical disk 14 b, as shown in FIG. 22( b), the secondlight beam L2 is incident on the diffraction optical element 311 as adiverging ray given by φ×Φinr=−0.048, and the light beam diffracted in afirst order diffraction direction (converging direction with respect tothe optical axis) on the diffracting face is refracted on the concaveface in the diverging direction so that the light beam is incident onthe objective lens 312 with a predetermined degree of divergence(φ×Φoutr=−0.1 in this Example). In this way, desirable light focusingcharacteristics are obtained for the second optical disk 14 b having a0.6 mm thick light transmissive layer.

Here, the provision of the diffraction optical element 311 enables thelight beam to be incident on the objective lens unit 313 with a smallerdegree of divergence than when it is incident on the objective lens 312,thereby reducing the adverse effect of radial shifting of the objectivelens unit 313.

For the third optical disk 14 c, as shown in FIG. 22( c), when the thirdlight beam L3 is incident on the diffraction optical element 311 as adiverging ray given by φ×ΦinIr=−0.18, the light beam is diffracted onthe diffracting face in a first order diffraction direction (convergingdirection with respect to the optical axis), and is refracted on theconcave face in a diverging direction so that the light beam is incidenton the objective lens 312 with a predetermined degree of divergence(φ×ΦoutIr=−0.2 in this Example). In this way, desirable light focusingcharacteristics are obtained for the third optical disk 14 c having a1.2 mm thick light transmissive layer.

Here, the provision of the diffraction optical element 311 enables thelight beam to be incident on the objective lens unit 312 with a smallerdegree of divergence than when it is incident on the objective lens 312,thereby reducing the adverse effect of radial shifting of the objectivelens unit 313.

FIG. 23( a), FIG. 23( b) and FIG. 23( c) represent changes in wavefrontaberration λrms on an image surface with respect to an amount ofshifting (objective shifting) of the objective lens unit 313 in theradial direction, as indicated by solid line, when the first, second andthird light beams are respectively focused on the first, second andthird disks 14 a, 14 b, and 14 c by using the optical pickup 300prepared in this Example. Further, broken line in FIG. 23( a), FIG. 23(b) and FIG. 23( c) indicates the results when an optical pickup(comparative example 1a) prepared for comparison was used. Note that,FIG. 23( a) shows the result when the first light beam was focused onthe first optical disk 14 a, and FIG. 23( b) shows the result when thesecond light beam was focused on the second optical disk 14 b, and FIG.23( c) shows the result when the third light beam was focused on thethird optical disk 14 c.

The comparative example 1a was prepared to optimize wavefront aberrationin such a manner that the first light beam is incident on the objectivelens as a parallel ray, and the second and third light beams areincident on the objective lens as predetermined diverging rays so as tocompensate for the spherical aberration caused by a thickness differenceof the light transmissive layers. Further, in order to optimizewavefront aberration, an aspherical lens is inserted in the opticalpaths of the diverging rays so as to prevent impairment of shiftingcharacteristics of the objective lens.

As can be seen from FIG. 23( a), the use of the optical pickup 300 ofthe present Example makes it possible to form a desirable focused lightspot on the first optical disk 14 a.

Further, as can be seen from FIG. 23( b), the use of the optical pickup300 of the present Example makes it possible to form a desirable focusedlight spot on the second optical disk 14 b. Compared with thecomparative example 1a, the present invention can more effectivelysuppress wavefront aberration, even though there is an area where theadverse effect of radial shifting of the objective lens unit 313, whichis caused when the second light beam is incident on the diffractionoptical element 311 as a diverging ray, is greater. Here, in order tofurther reduce the adverse effect of radial shifting of the objectivelens unit 313, the diffracting face 311 a of the diffraction opticalelement 311 on the side of the light source is made aspherical.

Further, as can be seen FIG. 23( c), the use of the optical pickup 300of the present Example makes it possible to form a desirable focusedlight spot on the third optical disk 14 c. The optical pickup 300 of thepresent Example can reduce the adverse effect of radial shifting of theobjective lens unit 313, which is caused when the third light beam isincident on the diffraction optical element 311 as a diverging ray, moreeffectively than the comparative example 1a. Here, in order to furtherreduce the adverse effect of radial shifting of the objective lens unit313, the diffracting face 311 a of the diffraction optical element 311on the side of the light source is made aspherical.

As described, for the first light beam L1, the optical pickup 300 of thepresent Example can attain the same level of wavefront aberration as canthe comparative example 1a. For the third light beam L3, the opticalpickup 300 can improve wavefront aberration more desirably than thecomparative example 1a. As for the second light beam L2, the waveformaberration can also be suppressed at low level, even though wavefrontaberration may be generated more than it is in the comparative example1a depending on the shift amount of the objective lens unit 313 in theradial direction.

FIG. 24 represents changes in wavefront aberration λrms with respect toshifting of the wavelength in the first light beam L1, as indicated bysolid line, when the first light beam is focused on the first disk 14 ausing the optical pickup 300 of the present Example. Further, brokenline in FIG. 24 indicates the result when an optical pickup (comparativeexample 1b) prepared for comparison was used. The comparative example 1bwas prepared to include an objective lens unit solely made up of theobjective lens 312 (objective lens 312 designated for the first lightbeam L1) used in the optical pickup 300 of the present Example. Itshould be noted here that the wavefront aberration for each wavelengthis the smallest wavefront aberration that provides the best focusingwith a given wavelength.

As shown in FIG. 24, the optical pickup 300 of the present Example has awider range of available wavelengths than the comparative example 1b.This is because the optical pickup 300 of the present Example includesthe diffraction optical element 311 made with the converging diffractiongrating and the planoconcave lens. The wavelength dependantcharacteristic can thus be improved over the case of solely using theobjective lens designated for the first light beam.

Generally, the high NA objective lens used for the next-generationhigh-density optical disk is made of glass with a high refractive index,and therefore has high wavelength dependency. Therefore, this type ofobjective lens causes difficulties in forming a desirable spot when afocal length is displaced by a large margin in response to wavelengthfluctuations caused by, for example, mode hopping, which cannot betracked by an actuator. On the other hand, the optical pickup 300 of thepresent example is capable of forming a desirable focused light spoteven in the presence of wavelength fluctuations caused by, for example,mode hopping.

Further, in the optical pickup 300 of the present Example, thediffracting face of the diffraction optical element 311 is designed suchthat the second order component of the diffracted light is used for thefirst light beam, and the first order component of the diffracted lightis used for the second and third light beams. Thus, as can be seen fromFIG. 19, the depth of the diffraction grating can be set so that all thefirst, second and third light beams can be used with a 90% or higherefficiency. In this way, an optical pickup that can record and eraseinformation requiring high power can easily be realized. In addition,the power of the light source can be reduced to suppress powerconsumption. Further, it also is possible to prevent unnecessary lightother than the diffracted light from entering the detector, therebysuppressing degradation of signals.

EXAMPLE 7

Another Example of the present invention is described below withreference to FIGS. 25( a),(b),(c), FIGS. 26( a), (b), (c), FIG. 27through FIG. 29.

In this Example, as shown in FIG. 25( a), FIG. 25( b) and FIG. 25( c),the first beam is incident on the diffraction optical element 311 with adegree of convergence φ×Φinb=0.06. Note that, as in Example 6 above,this Example also uses an objective lens 312 with an effective diameterφ=3 mm for the first light beam.

An optical pickup 300 of this Example is fabricated so that the firstlight beam is incident on the diffraction optical element 311 as aconverging ray given by φ×Φinb=0.06, while the second and third lightbeams are incident on the diffraction optical element 311 as asubstantial parallel ray and a diverging ray, which are given byφ×Φinr=0 and φ×ΦinIr=−0.15, respectively. The diffracting face of thediffraction optical element 311 is designed such that the second ordercomponent of the diffracted light is used for the first light beam, andthe first order component of the diffracted light is used for the secondand third light beams.

The diffraction optical element 311 is made with a concave face and adiffraction grating, and is disposed on the side of the light sourceopposite the aspherical objective lens 312. The concave face isspherical and has a curvature radius of 5 mm.

Here, the concave face is spherical because it is easier to fabricate.However, the concave face may be aspherical to improve the shiftingcharacteristic of the objective lens unit 313 in the radial direction.

In the optical pickup 300 of this Example, as shown in FIG. 25( a), forthe first optical disk 14 a, the first light beam L1 is incident on thediffraction optical element 311 as a converging ray given byφ×Φinb=0.06, and the light beam diffracted in a second order diffractiondirection (converging direction with respect to the optical axis) on thediffracting face is refracted in a diverging direction on the concaveface so that the light beam is incident on the objective lens 312 as aparallel ray and is focused on the first optical disk 14 a having a 0.1mm thick light transmissive layer. In this way, desirable light focusingcharacteristics are obtained.

For the second optical disk 14 b, as shown in FIG. 25( b), the secondlight beam L2 is incident on the diffraction optical element 311 as asubstantial parallel ray given by φ×Φinr=0, and the light beamdiffracted in a first order diffraction direction (converging directionwith respect to the optical axis) on the diffracting face is refractedin the diverging direction on the concave face so that the light beam isincident on the objective lens 312 with a predetermined degree ofdivergence (φ×Φoutr=−0.1 in this example). In this way, desirable lightfocusing characteristics are obtained for the second optical disk 14 bhaving a 0.6 mm thick light transmissive layer.

Here, the provision of the diffraction optical element 311 enables thelight beam to be incident on the objective lens 312 as a diverging rayand on the objective lens unit 313 as a parallel ray, thereby reducingthe adverse effect of radial shifting of the objective lens unit 313.

For the third optical disk 14 c, as shown in FIG. 25( c), when the thirdlight beam L3 is incident on the diffraction optical element 311 as adiverging ray given by φ×ΦinIr=−0.15, the light beam is diffracted atthe diffracting face in a first order diffraction direction (convergingdirection with respect to the optical axis), and is refracted on theconcave face in a diverging direction so that the light beam is incidenton the objective lens 312 with a predetermined degree of divergence(φ×ΦoutIr=−0.2 in this example). In this way, desirable light focusingcharacteristics are obtained for the third optical disk 14 c having a1.2 mm thick light transmissive layer.

Here, the provision of the diffraction optical element 311 enables thelight beam to be incident on the objective lens unit 313 with a smallerdegree of divergence than when it is incident on the objective lens 312,thereby reducing the adverse effect of radial of the objective lens unit313.

FIG. 26( a), FIG. 26( b) and FIG. 26( c) represent changes in wavefrontaberration λrms on an image surface with respect to an amount ofshifting (objective shifting) of the objective lens unit 313 in theradial direction, as indicated by solid line, when the first, second andthird light beams are respectively focused on the first, second andthird disks 14 a, 14 b, and 14 c by using the optical pickup 300prepared in this Example. Further, broken line in FIG. 26( a), FIG. 26(b) and FIG. 26( c) indicates the results when an optical pickup(comparative example 2a) prepared for comparison was used. Note that,FIG. 26( a) shows the result when the first light beam L1 was focused onthe first optical disk 14 a, and FIG. 26( b) shows the result when thesecond light beam L2 was focused on the second optical disk 14 b, andFIG. 26( c) shows the result when the third light beam L3 was focused onthe third optical disk 14 c.

The comparative example 2a was prepared to optimize wavefront aberrationin such a manner that the first light beam is incident on the objectivelens as a parallel ray, and the second and third light beams areincident on the objective lens as predetermined diverging rays so as tocompensate for the spherical aberration caused by the thicknessdifference of the light transmissive layers. Further, in order tooptimize wavefront aberration, an aspherical lens is inserted in theoptical paths of the diverging rays so as to prevent impairment ofshifting characteristics of the objective lens. Note that, thecomparative example 2a is identical with the foregoing comparativeexample 1a.

As can be seen from FIG. 26( a), the use of the optical pickup 300 ofthe present Example makes it possible to suppress the adverse effect ofradial shifting of the objective lens unit 313, in addition to forming adesirable focused light spot on the first optical disk 14 a.

Further, by causing the second light beam to be incident on thediffraction optical element 311 as a substantial parallel ray, theoptical pickup 300 of the present Example can suppress the adverseeffect of radial shifting of the objective lens unit 313 moreeffectively than the comparative example 2a, as shown in FIG. 26( b).

Further, as can be seen FIG. 23( c), compared with the comparativeexample 2a, the use of the optical pickup 300 of the present Examplemakes it possible to further reduce the adverse effect of radialshifting of the objective lens unit 313, which is caused when the thirdlight beam is incident on the diffraction optical element 311 as adiverging ray. Here, in order to further reduce the adverse effect ofradial shifting of the objective lens unit 313, the diffracting face 311a of the diffraction optical element 311 on the side of the light sourceis made aspherical.

As described, with the optical pickup 300 of the present example, thewavefront aberration caused on the first, second, and third opticaldisks 14 a, 14 b, and 14 c can be reduced more effectively than thecomparative example 2a.

FIG. 27 represents changes in wavefront aberration λrms with respect toshifting of the wavelength of the first light beam L1, as indicated bysolid line, when the first light beam L1 is focused on the first disk 14a using the optical pickup 300 of the present Example. Further, brokenline in FIG. 27 indicates the result when an optical pickup (comparativeexample 2b, identical with the comparative example 1b) prepared forcomparison was used. The comparative example 2b was prepared to includean objective lens unit solely made up of the objective lens 312(objective lens 312 designated for the first light beam) used in theoptical pickup 300 of the present Example. It should be noted here thatthe wavefront aberration for each wavelength is the smallest wavefrontaberration that provides the best focusing with a given wavelength.

As shown in FIG. 27, the optical pickup 300 of the present example has awider range of available wavelengths than the comparative example 2b.This is because the optical pickup 300 of the present Example includesthe diffraction optical element 311 made with the converging diffractiongrating and the planoconcave lens. The wavelength dependantcharacteristics can thus be improved over the case of solely using theobjective lens designated for the first light beam. Further, the opticalpickup 300 of the present Example is also able to form a desirablefocused light spot even in the presence of waveform fluctuations causedby, for example, mode hopping.

Further, in the optical pickup 300 of the present example, thediffracting face of the diffraction optical element 311 is designed suchthat the second order component of the diffracted light is used for thefirst light beam L1, and the first order component of the diffractedlight is used for the second and third light beams. Thus, as can be seenfrom FIG. 19, the depth of the diffraction grating can be set so thatall the first, second and third light beams can be used with a 90% orhigher efficiency. In this way, an optical pickup that can record anderase information requiring high power can easily be realized. Inaddition, the power of the light source can be reduced to suppress powerconsumption. Further, it also is possible to prevent unnecessary lightother than the diffracted light from entering the detector, therebysuppressing degradation of signals.

Here, FIG. 28 shows a relationship between power of the refracting faceof the diffraction optical element 311 for the first light beam andminimum grating pitch of the diffracting face, when the first light beamL1 is incident on the diffraction optical element 311 with a degree ofconvergence or divergence φ×Φinb=0.06. In the figure, solid line denotesa concave refracting face, and broken line denotes a convex refractingface. As FIG. 28 indicates, the minimum pitch of the diffracting facecan be increased when the refracting face is concave and when the powerof the refracting face has a small absolute value. A higher minimumpitch enables the diffraction optical element 311 to be fabricated moreeasily. Further, it becomes possible to reduce the aberration caused bydecentering of the diffracting face and the refracting face.

Further, FIG. 29 shows a relationship between power of the refractingface of the diffraction optical element 311 for the first light beam L1and wavefront aberration when the amount of radial shifting (objectiveshifting) of the objective lens unit 313 is 200 μm, when the first lightbeam L1 is incident on the diffraction optical element 311 with a degreeof convergence or divergence φ×Φinb=0.06. As FIG. 29 indicates, thewavefront aberration is generated abruptly when the power of therefracting face of the diffraction optical element 311 falls below −0.1.Accordingly, it is preferable that the power of the refracting face is−0.1 or greater.

EXAMPLE 8

The following will explain another Example of the present invention withreference to FIGS. 30( a), (b), (c), FIG. 31( a), (b), (c), and FIG. 32.

In this Example, as shown in FIG. 30( a), FIG. 30( b) and FIG. 30( c),the first light beam is incident on the diffraction optical element 311as a parallel ray.

The optical pickup 300 of the present example has such an arrangementthat the first light beam L1 is incident on the diffraction opticalelement 311 as a parallel ray, so that the adverse effect of radialshifting of the objective lens unit 313 can be eliminated almostcompletely for the first light beam L1 which requires the more accuratelight focusing characteristic. That is, the first light beam L1 isincident on the diffraction optical element 311 with a degree ofconvergence or divergence φ×Φinb=0. Note that, this Example uses anobjective lens 312 with an effective diameter φ=3 mm for the first lightbeam L1.

The optical pickup 300 of this Example is fabricated so that the firstand second light beams L1, L2 are incident on a diffraction opticalelement 311 as parallel rays, while the third light beam L3 is incidenton the diffraction optical element 311 as a diverging ray. Thediffracting face of the diffraction optical element 311 is designed suchthat the first order component of the diffracted light is used for thefirst light beam, and the zeroth order component of the diffracted lightis used for the second and third light beams.

The diffraction optical element 311 is made with a concave face 311 c(refracting face V) and a diffraction grating 311 a, and is disposed onthe side of the light source opposite the aspherical objective lens 312.The concave face 311 c is aspherical.

Here, since the concave face 311 c is aspherical, the sphericalaberration, which is caused by the thickness difference of the lighttransmissive layers of the second and third optical disks 14 b and 14 c,can be compensated for more effectively. In addition, the shiftingcharacteristics of the objective lens unit 313 in the radial directioncan be improved, thereby obtaining a desirable light focusingcharacteristic.

In the optical pickup 300 of this Example, as shown in FIG. 30( a), forthe first optical disk 14 a, the first light beam is incident on thediffraction optical element 311 as a parallel ray, and the light beam L1diffracted in a first order diffraction direction (converging directionwith respect to the optical axis) on the diffracting face of thediffraction grating 311 a is refracted in a diverging direction on theconcave face 311 c so that the light beam is incident on the objectivelens 312 as a parallel ray and is focused on the first optical disk 14 ahaving a 0.1 mm thick light transmissive layer. In this way, a desirablelight focusing characteristic is obtained.

For the second optical disk 14 b, as shown in FIG. 30( b), when thesecond light beam is incident on the diffraction optical element 311 asa substantial parallel ray, the second light beam, by not beingdiffracted on the diffracting face, is refracted in a divergingdirection on the concave face 311 c, so that the light beam is incidenton the objective lens 312 with a predetermined degree of divergence(φ×Φoutr=−0.03 in this Example). In this way, a desirable light focusingcharacteristic is obtained for the second optical disk 14 b having a 0.6mm thick light transmissive layer. Further, with the aspherical concaveface 311 c, persisting spherical aberrations can be compensated for, andimpairment of shifting characteristic of the objective lens unit 313 canbe suppressed, thereby obtaining more desirable light focusingcharacteristics.

Here, the provision of the diffraction optical element 311 enables thesecond light beam L2 to be incident on the objective lens 312 with apredetermined degree of divergence, even when the second light beam L2is incident on the objective lens unit 313 as a substantial parallelray, thereby reducing the adverse effect of radial shifting of theobjective lens unit 313.

For the third optical disk 14 c, as shown in FIG. 30( c), when the thirdlight beam L3 is incident on the diffraction optical element 311 as adiverging ray, the light beam that was not diffracted on the diffractingface is refracted in a diverging direction on the concave face 311 c, sothat the light beam is incident on the objective lens 312 with apredetermined degree of divergence (φ×ΦoutIr=−0.07 in this Example). Inthis way, a desirable light focusing characteristic is obtained for thethird optical disk 14 c having a 1.2 mm thick light transmissive layer.In addition, the spherical aberration that persists despite theaspherical concave face can be compensated for, and impairment ofshifting characteristics of the objective lens unit 313 can besuppressed, thereby obtaining desirable light focusing characteristics.

Here, the provision of the diffraction optical element 311 enables thelight beam to be incident on the objective lens unit 313 with a smallerdegree of divergence than when it is incident on the objective lens 312,thereby reducing the adverse effect of radial shifting of the objectivelens unit 313.

Further, in the optical pickup 300 of the present example, thediffracting face of the diffraction optical element 311 is designed suchthat the first order component of the diffracted light is used for thefirst light beam, and the zeroth order component of the diffracted lightis used for the second and third light beams. In this way, an opticalpickup that can record and erase information requiring high power caneasily be realized. In addition, the power of the light source can bereduced to suppress power consumption.

Further, in the present example, the provision of the diffractionoptical element 311 including the converging diffracting face and theconcave face enables a light beam to be incident on the objective lens312 with a predetermined degree of divergence even when the light beamis incident on the objective lens unit 313 as a weak diverging ray.Accordingly, the adverse effect of radial shifting of the objective lensunit 313 can be reduced. In addition, the semiconductor lasers 1 a, 1 b,and 1 c can be provided at a distant position from the objective lensunit 313, allowing the semiconductor lasers 1 a, 1 b, and 1 c to bedisposed more flexibly. Further, since the diffraction optical element311 is made with the converging diffracting face and the concaverefracting face, the optical pickup 300 of the present example has awider range of available wavelengths than the optical pickup designatedfor the first optical disk 14 a. As a result, the wavelength dependantcharacteristics can be improved over the case of solely using theobjective lens designated for the first optical disk 14 a. Thus, withthis configuration, desirable light focusing characteristics can bemaintained even in the presence of wavelength fluctuations caused by,for example, mode hopping. Further, it also possible to increase theminimum pitch of the diffracting face of the diffraction optical element311, enabling the diffraction optical element 311 to be easilyfabricated.

FIG. 31( a), FIG. 31( b) and FIG. 31( c) represent changes in wavefrontaberration λrms on an image surface with respect to an amount ofshifting (objective shifting) of the objective lens unit 313 in theradial direction, as indicated by solid line, when the first, second andthird light beams are respectively focused on the first, second andthird disks 14 a, 14 b, and 14 c using the optical pickup 300 preparedin this Example. Further, broken line in FIG. 31( a), FIG. 31( b) andFIG. 31( c) indicate the results when an optical pickup (“comparativeoptical pickup (I)” hereinafter) prepared for comparison was used. Notethat, FIG. 31( a) shows the result when the first light beam was focusedon the first optical disk 14 a, and FIG. 31( b) shows the result whenthe second light beam was focused on the second optical disk 14 b, andFIG. 31( c) shows the result when the third light beam was focused onthe third optical disk 14 c. Note that, as noted above, the term“spherical aberration” refers to an amount of aberration.

The comparative optical pickup was prepared to optimize wavefrontaberration in such a manner that the first light beam is incident on theobjective lens as a parallel ray, and the second and third light beamsare incident on the objective lens as predetermined diverging rays so asto compensate for the spherical aberration caused by the thicknessdifference of the light transmissive layers. Further, in order tooptimize wavefront aberration, an aspherical lens is inserted in theoptical paths of the diverging rays so as to prevent impairment ofshifting characteristics of the objective lens.

As can be seen from FIG. 31( a), the use of the optical pickup 300 ofthe present Example makes it possible to suppress the adverse effect ofradial shifting of the objective lens unit 313, in addition to forming adesirable focused light spot on the first optical disk 14 a.

Further, by causing the second light beam to be incident on thediffraction optical element 311 as a substantial parallel ray, theoptical pickup 300 of the present Example makes it possible to suppressthe adverse effect of radial shifting of the objective lens unit 313more effectively than the comparative optical pickup, as shown in FIG.31( b).

Further, as can be seen from FIG. 31( c), the use of the optical pickup300 of the present Example makes it possible to form a desirable lightspot on the third optical disk 14 c. Further, with the optical pickup300 of the present Example, the adverse effect of radial shifting of theobjective lens unit 313, which is caused when the third light beam isincident on the diffraction optical element 311 as a diverging ray, canbe reduced more effectively than the comparative optical pickup.

As described, for the first light beam, the optical pickup 300 of thepresent Example can attain the same level of wavefront aberration as canthe comparative optical pickup. For the second and third light beams,wavefront aberration can be improved more desirably over the comparativeexample 1a.

FIG. 32 represents changes in wavefront aberration λrms with respect toshifting of the wavelength of the first light beam, as indicated bysolid line, when the first light beam is focused on the first disk 14 ausing the optical pickup 300 of the present Example. Further, brokenline in FIG. 32 indicates the result when an optical pickup prepared forcomparison was used. The comparative optical pickup was prepared toinclude an objective lens unit solely made up of the objective lens 312(objective lens 312 designated for the first light beam) used in theoptical pickup 300 of the present Example. It should be noted here thatthe wavefront aberration for each wavelength is the smallest wavefrontaberration that provides the best focusing with a given wavelength.

As shown in FIG. 32, the optical pickup 300 of the present Example has awider range of available wavelengths than the comparative opticalpickup. Further, in response to wavelength fluctuations caused by, forexample, mode hopping that cannot be tracked by an actuator, the opticalpickup 300 of the present Example causes less aberration than theoptical pickup designated for the first optical disk 14 a. This isbecause the optical pickup 300 of the present example includes thediffraction optical element 311 made with the converging diffractiongrating 311 a and the planoconcave lens. The wavelength dependantcharacteristics can thus be improved over the case of solely using theobjective lens designated for the first light beam.

Generally, the high NA objective lens used for the next-generationhigh-density optical disk is made of glass with a high refractive index,and therefore has high wavelength dependency. Therefore, this type ofobjective lens causes difficulties in forming a desirable spot when afocal length is displaced by a large margin in response to wavelengthfluctuations caused by, for example, mode hopping that cannot be trackedby an actuator. On the other hand, the optical pickup 300 of the presentexample can form a desirable focused light spot even in the presence ofwavelength fluctuations caused by, for example, mode hopping.

EXAMPLE 9

The following will explain another Example of the present invention withreference to FIG. 32( a), (b), (c), FIG. 33( a), (b), (c), and FIG. 34.

In this Example, as shown in FIG. 33( a), FIG. 33( b) and FIG. 33( c),the first light beam is incident on the diffraction optical element 311as a parallel ray.

The optical pickup 300 of the present Example has such an arrangementthat the first light beam L1 is incident on the diffraction opticalelement 31 1 as a parallel ray, so that the adverse effect of radialshifting of the objective lens unit 313 can be eliminated almostcompletely for the first light beam L1 which requires a more accuratelight focusing characteristic. That is, the first light beam is incidenton the diffraction optical element 311 with a degree of convergence ordivergence φ×Φinb=0, as it is for Example 8. Note that, the presentexample uses an objective lens 312 with an effective diameter φ=3 mm forthe first light beam, as in Example 1.

The optical pickup 300 of this example is fabricated so that the firstand second light beams are incident on a diffraction optical element 311as parallel rays, while the third light beam is incident on thediffraction optical element 311 as a diverging ray. The diffracting faceof the diffraction optical element 311 is designed such that the firstorder component of the diffracted light is used for all of the first,second, third light beams.

The diffraction optical element 311 is made with a convex face 311 c(refracting face V) and a diffraction grating 311 a, and is disposed onthe side of the light source opposite the objective lens 312. The convexface is aspherical.

Here, with the aspherical convex face, shifting characteristics of theobjective lens unit 313 in the radial direction can be improved.

In the optical pickup 300 of this Example, as shown in FIG. 33( a), forthe first optical disk 14 a, the first light beam L1 is incident on thediffraction optical element 311 as a parallel ray, and the light beamdiffracted in a first order diffraction direction (converging directionwith respect to the optical axis) on the diffracting face of thediffraction grating 311 a is refracted in a diverging direction on theconvex face 311 c so that the light beam is incident on the objectivelens 312 as a parallel ray and is focused on the first optical disk 14 ahaving a 0.1 mm thick light transmissive layer. In this way, a desirablelight focusing characteristic is obtained.

For the second optical disk 14 b, as shown in FIG. 33( b), when thesecond light beam L2 is incident on the diffraction optical element 311as a substantial parallel ray, the light beam diffracted on thediffraction face of the diffraction grating 311 a in a first orderdiffraction direction (diverging direction with respect to the opticalaxis) is refracted in a diverging direction on the convex face 311 c, sothat the light beam is incident on the objective lens 312 with apredetermined degree of divergence (φ×Φoutr=−0.03 in this Example 9). Inthis way, a desirable light focusing characteristic is obtained for thesecond optical disk 14 b having a 0.6 mm thick light transmissive layer.In addition, the spherical aberration that persists despite theaspherical convex face 311 c can be compensated for, and impairment ofshifting characteristics of the objective lens unit 313 can besuppressed, thereby obtaining desirable light focusing characteristics.

Here, the provision of the diffraction optical element 311 enables thesecond light beam L2 to be incident on the objective lens 312 with apredetermined degree of divergence, even when the second light beam isincident on the objective lens unit 313 as a substantial parallel ray,thereby reducing the adverse effect of radial shifting of the objectivelens unit 313.

For the third optical disk 14 c, as shown in FIG. 33( c), when the thirdlight beam L3 is incident on the diffraction optical element 311 as adiverging ray, the light beam that was diffracted on the diffractionface of the diffraction grating 311 a in a first order diffractiondirection (diverging direction with respect to the optical axis) isrefracted in the converging direction on the convex face 311 c, so thatthe light beam is incident on the objective lens 312 with apredetermined degree of divergence (φ×ΦoutIr=−0.07 in this Example 9).In this way, a desirable light focusing characteristic is obtained withrespect to the third optical disk 14 c having a 1.2 mm thick lighttransmissive layer. In addition, the spherical aberration that persistsdespite the aspherical concave face 311 c can be compensated for, andimpairment of radial shifting characteristics of the objective lens unit313 can be suppressed, thereby obtaining desirable light focusingcharacteristics.

Here, the provision of the diffraction optical element 311 enables thelight beam to be incident on the objective lens unit 313 with a smallerdegree of divergence than when it is incident on the objective lens 312,thereby reducing the adverse effect of radial shifting of the objectivelens unit 313.

Further, in the optical pickup 300 of the present Example, thediffracting face of the diffraction optical element 311 is designed suchthat the first order component of the diffracted light is used for thefirst, second, and third light beams. In this way, an optical pickupthat can record and erase information requiring high power can easily berealized. In addition, the power of the light source can be reduced tosuppress power consumption.

Further, in the present Example, the provision of the diffractionoptical element 311 made with the diverging diffracting face and theconvex face enables a light beam to be incident on the objective lens312 with a predetermined degree of divergence even when the light beamis incident on the objective lens unit 313 as a weak diverging ray.Accordingly, the adverse effect of radial shifting of the objective lensunit 313 can be reduced. In addition, the semiconductor lasers 1 a, 1 b,and 1 c can be provided at a distant position from the objective lensunit 313, allowing the semiconductor lasers 1 a, 1 b, and 1 c to bedisposed more flexibly.

FIG. 34( a), FIG. 34( b) and FIG. 34( c) represent changes in wavefrontaberration λrms on an image surface with respect to an amount ofshifting (objective shifting) of the objective lens unit 313 in theradial direction, as indicated by solid line, when the first, second andthird light beams are respectively focused on the first, second andthird disks 14 a, 14 b, and 14 c using the optical pickup 300 preparedin this Example. Further, broken line in FIG. 34( a), FIG. 34( b) andFIG. 34( c) indicates the results when an optical pickup (“comparativeoptical pickup”) prepared for comparison was used. Note that, FIG. 34(a) shows the result when the first light beam was focused on the firstoptical disk 14 a, and FIG. 34( b) shows the result when the secondlight beam was focused on the second optical disk 14 b, and FIG. 34( c)shows the result when the third light beam was focused on the thirdoptical disk 14 c.

The comparative optical pickup was prepared to optimize wavefrontaberration in such a manner that the first light beam is incident on theobjective lens as a parallel ray, and the second and third light beamsare incident on the objective lens as predetermined diverging rays so asto compensate for the spherical aberration caused by the thicknessdifference of the light transmissive layers. Further, in order tooptimize wavefront aberration, an aspherical lens is inserted in theoptical paths of the diverging rays so as to prevent impairment ofshifting characteristics of the objective lens.

As can be seen from FIG. 34( a), the use of the optical pickup 300 ofthe present example makes it possible to suppress the adverse effect ofradial shifting of the objective lens unit 313, in addition to forming adesirable focused light spot on the first optical disk 14 a.

Further, by causing the second light beam to be incident on thediffraction optical element 311 as a substantial parallel ray, theoptical pickup 300 of the present Example makes it possible to suppressthe adverse effect of radial shifting of the objective lens unit 313more effectively than the comparative optical pickup, as shown in FIG.34( b).

Further, as can be seen FIG. 34( c), with the optical pickup 300 of thepresent example, the adverse effect of radial shifting of the objectivelens unit 313, which is caused when the third light beam is incident onthe diffraction optical element 311 as a diverging ray, can be reducedmore effectively than the comparative optical pickup.

As described, with the optical pickup 300 of the present example, thewavefront aberration caused on the second and third optical disks 14 band 14 c can be reduced more desirably over the comparative opticalpickup.

Note that, in the foregoing Examples, the first order component of thediffracted light is used for the first light beam, and the zeroth orderor first order component of the diffracted light is used for the secondand third beams. However, the second and third light beams are notnecessarily required to have the same diffraction order, provided thatthe diffraction orders of the second and third light beams are the sameor higher order than the diffraction order of the first light beam.

For example, the optical pickup 300 may be adapted so that the firstorder component is used for both the first and second light beams, andthe zeroth order component is used for the third light beam. In thisway, it is impossible to increase the efficiency of using light for eachof the first, second, and third light beams.

Fourth Embodiment

An optical pickup prepared for the present Embodiment has the sameschematic structure as that of the Third Embodiment illustrated in FIG.17. As in the Third Embodiment, the description of the presentembodiment will be given based on an optical pickup that is compatiblewith a next-generation high-density optical disk (first optical disk 14a, first recording medium), a conventional DVD (second optical disk 14b, second recording medium), and a conventional CD (third optical disk14 c, third recording medium).

Note that, the optical pickup according to the present Embodimentincludes an objective lens unit 413 shown in FIGS. 37( a), 37(b) and37(c), instead of the objective lens unit 313 of the optical pickup ofthe Third Embodiment. In the present Embodiment, the first optical disk14 a uses blue light (first light beam) of a short wavelength in thevicinity of 405 nm (first wavelength λ1), and has a light transmissivelayer with a thickness t1=0.6 mm. The second optical disk 14 b uses redlight (second light beam) of a long wavelength in the vicinity of 650 nm(second wavelength λ2), and has a light transmissive layer with athickness t2=0.6 mm. The third optical disk 14 c uses infrared light(third light beam) of a long wavelength in the vicinity of 780 nm (thirdwavelength λ3), and has a light transmissive layer with a thicknesst3=1.2 mm. Here, the objective lens is optimized for the first opticaldisk.

Further, in the present Embodiment, a wavelength-selective aperturefilter 410 controls aperture so that a numerical aperture NA1 (0.65 tobe specific), a numerical aperture NA2 (0.6 to be specific), and anumerical aperture NA3 (0.45 to be specific) are obtained for the lightbeams of the first, second and third wavelengths λ1, λ2, and λ3,respectively.

EXAMPLE 10

As shown in FIG. 37( a), FIG. 37( b) and FIG. 37( c), in this Example,the first light beam is incident on the diffraction optical element 411as a converging ray.

The optical pickup of this Example is fabricated so that the first lightbeam is incident on the diffraction optical element 411 as a convergingray, the second light beam as a converging or diverging ray, and thethird light beam as a diverging ray. The diffracting face of thediffraction optical element 411 is designed such that the second ordercomponent of the diffracted light is used for the first light beam, andthe first order component of the diffracted light is used for the secondand third light beams.

The diffraction optical element 411 is made with a concave face 411 band a diffraction grating 411 a, and is disposed on the side of thelight source opposite the objective lens 412. The concave face 411 b isspherical.

Here, the concave face 411 b is spherical because it is easier tofabricate. However, the concave face may be aspherical to improve theshifting characteristic of the objective lens unit 413 in the radialdirection.

In the optical pickup of this Example, as shown in FIG. 37( a), for thefirst optical disk 14 a, the first light beam L1 is incident on thediffraction optical element 411 as a converging ray, and the light beamdiffracted in a second order diffraction direction (converging directionwith respect to the optical axis) on the diffracting face of thediffraction grating 411 a is refracted in a diverging direction on theconcave face 411 b so that the light beam is incident on the objectivelens 412 as a parallel ray and is focused on the first optical disk 14 ahaving a 0.6 mm thick light transmissive layer. In this way, a desirablelight focusing characteristic is obtained.

For the second optical disk 14 b, as shown in FIG. 37( b), the secondlight beam is incident on the diffraction optical element 411 as aconverging or diverging ray, and the light beam diffracted in a firstorder diffraction direction (converging direction with respect to theoptical axis) on the diffraction face of the diffraction grating 411 ais refracted in a diverging direction on the concave face 411 b, so thatthe light beam is incident on the objective lens 412 with apredetermined degree of divergence. In this way, a desirable lightfocusing characteristic is obtained for the second optical disk 14 bhaving a 0.6 mm thick light transmissive layer. As used herein, “apredetermined degree of divergence” is the extent to which the incidentlight beam on the objective lens needs to be diverged in order tocompensate for the spherical aberration caused by the differences in thewavelengths.

For the third optical disk 14 c, as shown in FIG. 37( c), when the thirdlight beam is incident on the diffraction optical element 411 as adiverging ray, the light beam that was diffracted in a first orderdiffraction direction (converging direction with respect to the opticalaxis) is refracted in the diverging direction on the concave face 411 b,so that the light beam is incident on the objective lens 412 with apredetermined degree of divergence. In this way, a desirable lightfocusing characteristic is obtained with respect to the third opticaldisk 14 c having a 1.2 mm thick light transmissive layer. As usedherein, “a predetermined degree of divergence” is the extent to whichthe incident light beam on the objective lens needs to be diverged inorder to compensate for the spherical aberration caused by thedifferences in the wavelengths or the differences in thickness of thelight transmissive layers.

Here, the provision of the diffraction optical element 411 enables thelight beam to be incident on the objective lens unit 413 with a smallerdegree of divergence than when it is incident on the objective lens 412,thereby reducing the adverse effect of radial shifting of the objectivelens unit 413.

FIG. 38( a), FIG. 38( b) and FIG. 38( c) represent changes in wavefrontaberration λrms on an image surface with respect to an amount ofshifting (objective shifting) of the objective lens unit 413 in theradial direction, as indicated by solid line (A), when the first, secondand third light beams are respectively focused on the first, second andthird disks 14 a, 14 b, and 14 c using the optical pickup prepared inthis Example. Further, broken line (B) in FIG. 38( a), FIG. 38( b) andFIG. 38( c) indicates the results when an optical pickup (“comparativeexample”) prepared for comparison was used. Note that, FIG. 38( a) showsthe result when the first light beam was focused on the first opticaldisk 14 a, and FIG. 38( b) shows the result when the second light beamwas focused on the second optical disk 14 b, and FIG. 38( c) shows theresult when the third light beam was focused on the third optical disk14 c.

The wavefront aberrations of the comparative example are the resultsobtained when the first light beam was incident on the objective lens asa parallel ray, and when the second and third light beams were incidenton the objective lens as predetermined diverging rays in order tocompensate for the chromatic aberration or spherical aberration causedby the differences in the wavelengths or the differences in thickness ofthe light transmissive layers.

As can be seen from FIG. 38( a), the use of the optical pickup of thepresent example makes it possible to suppress the adverse effect ofradial shifting of the objective lens unit 413, in addition to forming adesirable focused light spot on the first optical disk 14 a.

Further, as can be seen from FIG. 38( b), the use of the optical pickupof the present example makes it possible to form a desirable focusedlight spot on the second optical disk 14 b. The extent of wavefrontaberration is less in the present Example than in the comparativeexample, even though there is an area where the adverse effect of radialshifting of the objective lens unit 413, which is caused when the secondlight beam is incident on the diffraction optical element 411 as adiverging ray, is greater.

Further, as can be seen FIG. 38( c), with the optical pickup of thepresent example, the adverse effect of radial shifting of the objectivelens unit 413, which is caused when the third light beam is incident onthe diffraction optical element 411 as a diverging ray, can be reducedmore effectively than the comparative example.

As described, with the optical pickup of the present Example, thewavefront aberration caused on the first, second and third optical disks14 a, 14 b and 14 c can be reduced more desirably over the comparativeexample.

FIG. 39 represents changes in wavefront aberration λrms with respect toshifting of the wavelength of the first light beam, as indicated bysolid line (A), when the first light beam is focused on the first disk14 a using the optical pickup of the present Example. Further, brokenline (B) in FIG. 32 indicates the result when an optical pickup(comparative example) prepared for comparison was used. The comparativeexample was prepared to include an objective lens unit solely made up ofthe objective lens 412 (objective lens 412 designated for the firstlight beam) used in the optical pickup of the present Example. It shouldbe noted here that the wavefront aberration for each wavelength is thesmallest wavefront aberration that provides the best focusing with agiven wavelength (λ=405 in the example).

As shown in FIG. 39, the optical pickup of the present Example has awider range of available wavelengths than the comparative example. Thisis because the optical pickup of the present Example includes thediffraction optical element 411 made with the converging diffractiongrating and the planoconcave lens. The wavelength dependantcharacteristics can thus be improved over the case of solely using theobjective lens designated for the first light beam. Further, with theoptical pickup of the present Example, a desirable focused light spotcan be formed even in the presence of wavelength fluctuations caused by,for example, mode hopping.

Further, in the optical pickup of the present Example, the diffractingface of the diffraction optical element 411 is designed such that thesecond order component of the diffracted light is used for the firstlight beam, and the first order component of the diffracted light isused for the second and third light beams. Thus, as can be seen fromFIG. 39, the depth of the diffraction grating can be set so that all ofthe first, second and third light beams can be used with a 90% or higherefficiency. In this way, an optical pickup that can record and eraseinformation requiring high power can easily be realized. In addition,the power of the light source can be reduced to suppress powerconsumption. Further, it also is possible to prevent unnecessary lightother than the diffracted light from entering the detector, therebysuppressing degradation of signals.

As has been described, the optical pickup of the present invention iscapable of recording and reproducing information with respect todifferent types of recording media whose light transmissive layers havedifferent thicknesses, and whose optimum light beam wavelengths forreproduction are different. The optical pickup can also suppressimpairment of light focusing characteristics, which occurs when theobjective lens shifts in the radial direction, for an optical click (CDto be specific) having a thick light transmissive layer requiring alarge degree of divergence for the incident light on the objective lens.The optical pickup of the present invention can achieve these effectswith a simple structure.

The optical pickup of any of the foregoing embodiments is applicable to,for example, an information recording/reproducing apparatus illustratedin FIG. 46. FIG. 46 is a block diagram schematically showing theinformation recording/reproducing apparatus using the optical pickup 300shown in FIG. 17.

As illustrated in FIG. 46, the information recording/reproducingapparatus of the embodiment has the configuration as described below, inaddition to the optical pickup 300 shown in FIG. 17.

The reproduced signal detecting optical systems 15 a, 15 b, and 15 c areconnected to a demodulating circuit 19 and an error detecting circuit17. The error detecting circuit 17 is connected to a driving circuit 18that drives a tracking control and a focus control mechanism for theobjective lens unit 313. A photodetector supplies an electrical signalaccording to a light spot image to the demodulating circuit 19 and theerror detecting circuit 17. The demodulating circuit 19 generates arecording signal based on the electrical signal. Based on the electricalsignal, the error detecting circuit 17 generates various signals such asa focus error signal, a tracking error signal, and a servo signal, andsupplies these driving signals via the driving circuit 18 so as to drivethe objective lens unit 313 and other elements according to the drivingsignals.

An optical pickup unit 25 includes the optical pickup 300 shown inFIG. 1. The optical pickup 300 is coupled to semiconductor laser drivingcircuits 21 a, 21 b, and 21 c for respectively driving semiconductorlasers 1 a, 1 b, and 1 c, which are light sources for emitting the lightof the first, second, and third wavelengths, respectively.

An input/output terminal 23 is connected to the modulating/demodulatingcircuit 19, so as to input recording data from the central devices tothe modulating/demodulating circuit 19, and to output the reproduceddata of the information recording/reproducing apparatus from themodulating/demodulating circuit 19 to the central devices.

A control signal input/output terminal 24 is connected to a controlcircuit 20, so as to input a control signal from the central devices tothe control circuit 20, and to output the control result of theinformation recording/reproducing apparatus to the central devices. Thecontrol circuit 20 is connected to the error detecting circuit 17, themodulating/demodulating circuit 19, and a switching circuit 22, so as tocontrol recording and reproducing operations based on the control signalfrom the control signal input/output terminal 24.

The modulating/demodulating circuit 19 supplied recording signalsaccording to recording data to the semiconductor laser driving circuits21 a, 21 b, and 21 c, and receives reproduced signals from thereproduced signal detecting optical systems 15 a, 15 b, and 15 c. Theerror detecting circuit 17 receive the reproduced signals from thereproduced signal detecting optical systems 15 a, 15 b, and 15 c. Basedon the reproduced signals, the error detecting circuit 17 generatesvarious signals such as a focus error signal, a tracking error signal,and a servo signal, and outputs these driving signals to the drivingcircuit 18. Based on the driving signals from the error detectingcircuit 17, the driving circuit 18 drives the optical pickup unit 25 byservo control.

Under the instructions of the control circuit 20, the switching circuit22 switches between (a) the semiconductor laser driving circuit 21 a andthe reproduced signal detecting optical system 15 a for the first lightbeam, (b) the semiconductor laser driving circuit 21 b and thereproduced signal detecting optical system 15 b for the second lightbeam, and (c) the semiconductor laser driving circuit 21 c and thereproduced signal detecting optical system 15 c for the third lightbeam. In addition, the switching circuit 22 supplies power to therespective semiconductor lasers for recording and reproducing. Here, inthe case where the stray light entering the reproduced signal detectingoptical systems 15 a, 15 b, and 15 c are essentially negligible, theswitching circuit 22 is not required to switch between the reproducedsignal detecting optical systems 15 a, 15 b, and 15 c.

Referring to FIG. 46, the recording and reproducing operations aredescribed below.

In recording, the central devices supply recording data to theinput/output terminal 23, while the control input/output terminal 24receives a recording control signal and a light beam switching signalaccording to the type of optical disk inserted in the informationrecording/reproducing apparatus. The following assumes that the inputlight beam switching signal is the first light beam L1.

Under the instructions of the control circuit 20, the switching circuit22 turns ON the semiconductor laser driving circuit 21 a and thereproduced signal detecting optical system 15 a for the first light beamL1, while the other semiconductor laser driving circuits (21 b, 21 c)and the reproduced signal detecting optical systems (15 b, 15 c) remainOFF. In addition, the switching circuit 22 drives the semiconductorlaser driving circuit 21 a, so as to drive the semiconductor laser 1 awith a recording power stronger than the reproducing power.

Under the instructions of the control circuit 20, themodulating/demodulating circuit 19 outputs a recording signal from therecording data supplied from the input/output terminal 23. The recordingsignal is supplied to the semiconductor laser driving circuit 1 a, so asto project the first light beam L1 on the first optical disk 14 aaccording to the recording signal. The error detecting circuit 17 viathe reproduced signal detecting optical system 15 a receives an outputsignal according to a light spot image formed by the projection of thefirst light beam L1 on the first optical disk 14 a. Based on the outputsignal, the error detecting circuit 17 supplies the driving signals tothe driving circuit 18 under the instructions of the control circuit 20,so as to servo control the optical pickup unit 25. In this manner, theinformation recording/reproducing apparatus of the embodiment drives theoptical pickup unit 25 by servo control in such a manner that the firstlight beam L1 according to the recording signal is projected on thefirst optical disk 14 a with a recording power, so that the recordingdata from the central devices are recorded on the first optical disk 14a.

In reproducing, the control input/output terminal 24 receives areproducing control signal and a light beam switching signal accordingto the type of optical disk inserted in the informationrecording/reproducing apparatus. The following assumes that the inputlight beam switching signal is the first light beam L1.

Under the instructions of the control circuit 20, the switching circuit22 turns ON the semiconductor laser driving circuit 21 a and thereproduced signal detecting optical system 15 a for the first light beamL1, while the other semiconductor laser driving circuits (21 b, 21 c)and the reproduced signal detecting optical systems (15 b, 15 c) remainOFF. In addition, the switching circuit 22 drives the semiconductorlaser driving circuit 21 a, so as to drive the semiconductor laser 1 awith a reproducing power weaker than the recording power.

When the semiconductor laser 1 a projects the first light beam L1 of areproducing power on the first optical disk 14 a, the reproduced signaldetecting optical system 15 a supplies an output signal according to alight spot image formed on the first optical disk to themodulating/demodulating circuit 19 and the error detecting circuit 17.Based on the output signal, the error detecting circuit 17 supplies thedriving signals to the driving circuit 18 under the instructions of thecontrol circuit 20, so as to servo control the optical pickup unit 25.Based on the output signal, the modulating/demodulating circuit 19outputs reproduced data to the input/output terminal 23 and to thecentral devices, under the instructions of the control circuit 20.

In this manner, the information recording/reproducing apparatus of theembodiment drives the optical pickup unit 25 by servo control in such amanner that the first light beam L1 is projected on the first opticaldisk 14 a with a reproducing power, so as to reproduce the recordedsignal in the first optical disk 14 a and output the reproduced data tothe central devices.

In the case where the input light beam switching signal is the secondlight beam L2, the switching circuit 22 turns ON the semiconductor laserdriving circuit 21 b and the reproduced signal detecting optical system15 b. In the same manner, when the input light beam switching signal isthe third light beam L3, the switching circuit 22 turns ON thesemiconductor laser driving circuit 21 c and the reproduced signaldetecting optical system 15 c. That is, when the inserted disk is thesecond optical disk, the semiconductor laser driving circuit 21 b andthe reproduced signal detecting optical system 15 b for the second lightbeam L2 are used to record or reproduce information with respect to thesecond optical disk. When the inserted disk is the third optical disk,the semiconductor laser driving circuit 21 c and the reproduced signaldetecting optical system 15 c for the third light beam L3 are used torecord or reproduce information with respect to the third optical disk.

FIG. 46 described the information recording/reproducing apparatus withthree light sources. However, not limiting to this, an optical pickupwith two light sources, for example, such as the optical pickup 100shown in FIG. 1, is also applicable to the informationrecording/reproducing apparatus.

The embodiments and concrete examples of implementation discussed in theforegoing detailed explanation serve solely to illustrate the technicaldetails of the present invention, which should not be narrowlyinterpreted within the limits of such embodiments and concrete examples,but rather may be applied in many variations within the spirit of thepresent invention, provided such variations do not exceed the scope ofthe patent claims set forth below.

The present invention provides an optical pickup for recording orreproducing information with respect to a first recording medium havinga light transmissive layer of a thickness t1 on an information recordingface, the optical pickup recording or reproducing information by forminga first light spot on the information recording face by focusing a firstlight beam of a wavelength λ1 on the information recording face, theoptical pickup including: a diffraction optical element including adiffracting face and a refracting face for diffracting and refractingthe first light beam so as to emit the first light beam; an objectivelens for causing a diffracted ray of a predetermined diffraction orderof the first light beam emitted from the diffraction optical element tofocus on the information recording face of the first recording medium soas to form the first light spot; and a collimator lens, provided betweenthe first light source and the diffraction optical element, for causingthe first light beam from the first light source to be incident on thediffraction optical element as a parallel ray, the diffracting face ofthe diffraction optical element having such a diffraction characteristicthat the first light beam is diffracted toward an optical axis, and therefracting face being a concave face.

With this configuration, shifting of a focal point caused by wavelengthfluctuations can be suppressed more effectively, even when the opticalpickup is specifically designated for the next-generation high-densityoptical disk.

It is preferable in the optical pickup of the present invention that thediffraction optical element satisfiesΦ=Φ_(D)+Φ_(L)=0where Φ is the power of the diffraction optical element, Φ_(D) is thepower of the diffracting face of the diffraction optical element, andΦ_(L) is the power of the refracting face of the diffraction opticalelement.

With this configuration, the first light beam that emerges from thefirst optical system as a parallel ray can emerge from the diffractionoptical element also as a parallel ray after the passage through thediffraction optical element. In this way, the aberration caused bymisalignment with the objective lens can be suppressed. Here, thediffraction optical element may be disposed anywhere between the firstoptical system and the objective lens.

The present invention provides an optical pickup for recording orreproducing information with respect to (a) a first recording mediumhaving a light transmissive layer of a thickness t1 on an informationrecording face and (b) a second recording medium having a lighttransmissive layer of a thickness t2 greater than t1 on an informationrecording face, the optical pickup recording or reproducing informationwith respect to the first recording medium by forming a first light spoton the information recording face of the first recording medium byfocusing a first light beam of a wavelength λ1 on the informationrecording face, and the optical pick up recording or reproducinginformation with respect to the second recording medium by forming asecond light spot on the information recording face of the secondrecording medium by focusing a second light beam of a wavelength λ2greater than λ1 on the information recording face, the optical pickupincluding: a diffraction optical element for diffracting and refractingincident rays of the first and second light beams so as to emit thefirst and second light beams; and an objective lens for causingrespective diffracted rays of predetermined diffraction orders of thefirst and second light beams emitted from the diffraction opticalelement to focus on the respective information recording faces of thefirst and second recording media so as to form the first and secondlight spots, the first light beam and the second light beam beingincident on the diffraction optical element as light beams withdifferent degrees of convergence or divergence.

In the configuration where diffracted rays of the first and second lightbeams of different wavelengths are focused through the common objectivelens to form the first and second light spots on the respectiveinformation recording faces of the first and second recording mediarespectively having light transmissive layers of different thicknesses,the degrees of convergence and/or divergence of the respectivediffracted rays of the first and second light beams incident on theobjective lens must provide a sufficiently large angle difference inorder to sufficiently reduce the wavefront aberrations in the respectivediffracted rays of the focused light and to thereby obtain a desirablelight focusing characteristic.

Causing the first and second light beams to be incident on thediffraction optical element as light beams with different degrees ofconvergence and/or divergence helps to increase the angle difference.This enables the required diffracting and refracting characteristics forthe diffraction optical element to be set more freely, allowing for moreflexible design for the diffraction optical element. As a result, usingthe diffraction optical element, which is easy to fabricate, it ispossible to realize the optical pickup which can sufficiently reducewavefront aberration in the diffracted rays of the focused light.

Note that, when the first and second light beams enter the diffractionoptical element as light beams with different degrees of convergence ordivergence, the incident ray of one of the first and second light beamsmay be a converging ray while the other is a diverging ray.Alternatively, the incident ray of one of the first and second lightbeams may be a parallel ray while the other is a converging ray or adiverging ray. Further, the incident rays of both the first light beamand the second light beams may be converging rays or diverging rays withdifferent degrees of convergence or divergence.

It is preferable in the optical pickup of the present invention that thediffraction optical element includes a converging diffraction gratingand a diverging lens.

With this configuration, the wavefront aberration in response towavelength fluctuations can be suppressed, and a desirable lightfocusing characteristic can be obtained even in the presence ofwavelength fluctuations.

It is preferable in the optical pickup of the present invention that thefirst light beam is incident on the diffraction optical element as aconverging ray, and the second light beam is incident on the diffractionoptical element as a diverging ray.

With this configuration, the wavefront aberration can be suppressed overa relatively wide shift range, even when the diffraction optical elementand the objective lens shift in the direction of the optical axes of thefirst and second light beams.

It is preferable in the present invention that the objective lens causesthe second order component of the diffracted light for the first lightbeam emitted from the diffraction optical element and the first ordercomponent of the diffracted light for the second light beam emitted fromthe diffraction optical element to focus on the respective informationrecording faces of the first and second recording media so as to formthe first and second light spots.

It is also preferable in the present invention that the objective lenscauses the third order component of the diffracted light for the firstlight beam emitted from the diffraction optical element and the secondorder component of the diffracted light for the second light beamemitted from the diffraction optical element to focus on the respectiveinformation recording faces of the first and second recording media soas to form the first and second light spots.

With this configuration, among the diffracted rays produced by thediffraction optical element, those with high diffraction efficiency canbe used. This enables the first and second light beams to be efficientlyused, allowing high power laser beams to be projected onto the first andsecond recording media without increasing the power consumed by thelight sources for the first and second light beams.

The present invention provides an optical pickup for recording orreproducing information with respect to the first, second, and thirdrecording media having information recording faces and lighttransmissive layers, the light transmissive layers of the first, second,and third recording media being formed on the respective informationrecording faces and respectively having thicknesses t1, t2, and t3,which are related to one another by t1<t2<t3, the optical pickuprecording or reproducing information by focusing first, second, andthird light beams of wavelengths λ1, λ2, and λ3, which are related toone another by λ1<λ2<λ3, on the respective information recording faces,the optical pickup including: an objective lens, movable in asubstantially orthogonal direction with respect to respective opticalaxes of the first, second, and third light beams, for focusing thefirst, second, and third light beams on the respective informationrecording faces of the first, second, and third recording media; and adiffraction optical element, provided on an incident side of the first,second, and third light beams and movable with the objective lens, fordiffracting and refracting the first, second, and third light beams soas to cause the first, second, and third light beams to be incident onthe objective lens as diffracted rays of predetermined diffractionorders, the diffraction optical element causing the second and thirdlight beams to be incident on the objective lens as diverging rays, andthe diffraction optical element satisfying|Φinr|<|Φoutr|, and |ΦinIr|<|ΦoutIr|where Φinr and ΦinIr are degrees of convergence and/or divergence ofincident rays of the second and third light beams, respectively, on thediffraction optical element, and Φoutr and ΦoutIr are degrees ofconvergence and/or divergence of incident rays of the second and thirdlight beams, respectively, on the objective lens.

In the configuration of the optical pickup recording or reproducing thefirst, second, and third optical disks using the first, second, andthird light beams of different wavelengths, the objective lens usedcompensates for the aberration that is caused when the first light beamof the shortest wavelength is focused on the first recording medium.

Using the objective lens to focus the second and third light beams onthe second and third recording media respectively having lighttransmissive layers of different thicknesses from that of the firstrecording medium increases spherical aberration in the second and thirdlight beams. Such an increase of spherical aberration can be suppressedby compensating for the aberration by generating aberration of theopposite direction. This can be carried out by causing the second andthird light beams to be incident on the objective lens as divergingrays.

Here, in order to sufficiently reduce the spherical aberration, thesecond and third light beams must enter the objective lens with largedegrees of divergence. However, increasing the degrees of divergence ofincident light beams on the objective lens increases coma aberrationthat affects the aperture spot on the recording medium when theobjective lens moves in the radial direction (direction substantiallyorthogonal to the optical axes of the first, second, and third lightbeams incident on the objective lens) during tracking or otheroperations, with the result that the light focusing characteristic isgreatly impaired.

To avoid this problem, the foregoing configuration uses the diffractionoptical element that is movable with the objective lens, so as to causethe second and third light beams to be incident on the objective lens asdiverging rays. The diffraction optical element functions to satisfy|Φinr|<|Φoutr|, and |ΦinIr|<|ΦoutIr|.That is, the degrees of convergence and/or divergence have largerabsolute values for the second and third light beams emerging from thediffraction optical element than for the second and third light beamsincident on the diffraction optical element.

This enables the second and third light beams to be incident on the unitmade up of the objective lens and the diffraction optical element(objective lens unit) with degrees of convergence and/or divergence ofsmall absolute values. That is, the second and third light beams can bemade incident as near parallel rays. As a result, the foregoingconfiguration is able to suppress impairment of the light focusingcharacteristic caused by radial shifting (objective shifting) of theobjective lens unit, more effectively than the configuration without thediffraction optical element.

Thus, with the foregoing configuration, a single objective lens is usedto form desirable light spots on the recording media with lighttransmissive layers of different thicknesses, so as to record orreproduce information. In addition, the light focusing characteristic isnot severely impaired even when the objective lens unit shifts in theradial direction.

It is preferable in the optical pickup of the present invention that thediffraction optical element causes the first light beam to be incidenton the objective lens as a parallel ray.

With this configuration, by causing the first light beam, which requiresthe most accurate light focusing characteristic, to be incident on theobjective lens unit as a parallel ray, the aberration due tomisalignment of the diffraction optical element with the objective lenscan be suppressed when using the first light beam.

It is preferable in the optical pickup of the present invention that thediffraction optical element according to the foregoing configurationcauses the first, second, and third light beams to be incident on theobjective lens as diffracted rays of the second order, first order, andfirst order, respectively, the diffraction optical element having thehighest diffraction efficiency for the second order component of thediffracted light for the first light beam, the first order component ofthe diffracted light for the second light beam, and the first ordercomponent of the diffracted light for the third light beam.

With this configuration, diffraction efficiency can be improved for allof the first, second, and third light beams. This enables the power ofthe light source producing each light beam to be reduced, therebyreducing the power consumption of the light source. The foregoingconfiguration is particularly effective for the optical pickup thatrecords or erases information requiring high power beams. Preferably,the diffraction optical element produces the second order component ofthe diffracted light with a diffraction efficiency of 90% or greater forthe first light beam.

It is preferable in the optical pickup of the present invention that thethird light beam according to the foregoing configuration is incident onthe diffraction optical element as a diverging ray.

With this configuration, by causing the third light beam, which requiresthe largest degree of divergence, to be incident on the diffractionoptical element as a diverging ray in order to suppress sphericalaberration, the first and second light beams can be made incident on thediffraction optical element with small degrees of divergence.

It is preferable in the optical pickup of the present invention that thefirst and second light beams according to the foregoing configurationare incident on the diffraction optical element as a parallel ray and adiverging ray, respectively.

With this configuration, the second and third light beams can be madeincident on the diffraction optical element with relatively smalldegrees of divergence (see FIG. 20), while the first light beam isincident on the diffraction optical element as a parallel ray. As aresult, impairment of the light focusing characteristic caused by radialshifting of the objective lens unit can be suppressed more effectively.

It is preferable in the optical pickup of the present invention that thefirst light beam according to the foregoing configuration is incident onthe diffraction optical element as a converging ray, and the secondlight beam is incident on the diffraction optical element as aconverging, parallel, or diverging ray.

With this configuration, the first, second, and third light beams can bemade incident on the diffraction optical element with degrees ofconvergence and/or divergence of relatively small absolute values (seeFIG. 20). As a result, impairment of the light focusing characteristiccaused by radial shifting of the objective lens unit can be suppressedmore effectively.

For example, it is preferable that the first, second, and third lightbeams are incident on the diffraction optical element with degrees ofconvergence and/or divergence that satisfy

0 ≤ ϕ × Φ inb ≤ 0.11 − 0.048 ≤ ϕ × Φ inr ≤ 0.04 − 0.18 ≤ ϕ × Φ inIr ≤ −0.1where φ is an effective diameter of the objective lens for the firstlight beam.

In other words, the diffraction optical element should preferablysatisfy−0.11≦φ×Φb≦0−0.2≦φ×Φr≦−0.002−0.16≦φ×ΦIr≦0.03where Φb, Φr, ΦIr are powers of the diffraction optical element for thefirst, second, and third light beams, respectively.

It is preferable in the optical pickup of the present invention that thediffraction optical element includes a diverging diffracting face and aconcave refracting face.

With this configuration, the range of available wavelengths can be madewider than that for the optical pickup designated for the firstrecording medium, thereby improving the wavelength dependentcharacteristic over the case of solely using the objective lensdesignated for the first recording medium. Thus, with the foregoingconfiguration, a desirable light focusing characteristic can bemaintained even in the presence of wavelength fluctuations caused by,for example, mode hopping. Further, the minimum pitch on the diffractingface of the diffraction optical element can be increased, making iteasier to fabricate the diffraction optical element.

It is preferable in the present invention that the diffraction opticalelement according to the foregoing configuration has a sphericalrefracting face.

With this configuration, the diffraction optical element can befabricated more easily, providing the optical pickup inexpensively.

It is preferable in the optical pickup of the present invention that thediffraction optical element according to the foregoing configuration hasa refracting face whose power is not less than −0.1 for the first lightbeam.

With this configuration, impairment of the light focusing characteristiccaused by radial shifting of the objective lens unit can be suppressedmore effectively.

The present invention provides an optical pickup including first,second, and third light sources (for example, semiconductor lasers) forrespectively emitting first, second, and third light beams ofwavelengths λ1, λ2, and λ3, which are related to one another byλ1<λ2<λ3, the optical pickup recording or reproducing information withrespect to first, second, and third recording media respectively havinglight transmissive layers of different thicknesses, by focusing thefirst, second, and third light beams on respective information recordingfaces of the first, second, and third recording media using commonfocusing means (for example, objective lens), the optical pickup furtherincluding: a diffraction optical element, provided in a common opticalpath between the first, second, and third light sources and an objectivelens and including a diffracting face and a refracting face, for causingthe first, second, and third light beams to diverge or convergeaccording to the wavelengths of the first, second, and third lightbeams, and causing the first, second, and third light beams to diffracton the diffracting face so that the first light beam is incident on thefocusing means as a diffracted ray of the first order and the second andthird light beams are incident on the focusing means as diffracted raysof the first or lower order, the diffracting face of the diffractionoptical element having a diffraction grating whose depth is set so thatdiffraction efficiency for one diffraction order is higher than that forother diffraction orders with respect to the diffracted rays of each ofthe first, second, and third light beams incident on the focusing means.

In the configuration of the optical pickup recording or reproducing thefirst, second, and third recording media using the first, second, andthird light beams of different wavelengths, the focusing means usedcompensates for the aberration caused when the first light beam of theshortest wavelength is focused on the first recording medium. However,directly using the same focusing means to focus the second and thirdlight beams on the second and third recording media respectively havinglight transmissive layers of different thicknesses from that of thefirst recording medium increases the spherical aberration in the secondand third light beams.

In the foregoing configuration, however, by the provision of thediffraction optical element in the optical path between the first,second, and third light sources and the objective lens, the respectivelight spots of the focused first, second, and third light beams ofdifferent wavelengths through the focusing means can be formed indifferent positions on the respective information recording faces of thefirst, second, and third recording media having the light transmissivelayers of different thicknesses. Thus, with the foregoing configuration,a single focusing means can be used to focus the first, second, andthird light beams of different wavelengths on the respective informationrecording faces of the first, second, and third recording mediarespectively having light transmissive layers of different thicknesses.In addition, the degrees of divergence with respect to the diffractionoptical element can be reduced for the second and third light beams,making it possible to suppress the adverse effect of radial shifting ofthe focusing means during tracking or other operations.

The diffraction efficiency of the diffraction optical element for thediffracted rays of the respective light beams are determined by thedepth of the diffraction grating on the diffracting face of thediffraction optical element. The inventors of the present invention havefound that a loss of light quantity of incident light on the respectiverecording media can be reduced for any of the diffracted rays when thediffracting face of the diffraction optical element is so designed thatthe first order component of the diffracted light is used for the firstlight beam, and the first or lower order component of the diffractedlight is used for the second and third light beams, and when the depthof the diffraction grating on the diffracting face is set so that thediffraction efficiency for one diffraction order is higher than that forother diffraction orders with respect to the diffracted light of each ofthe first, second, and third light beams incident on the focusing means.As a result, an optical pickup is realized that is compatible with threekinds of recording media with light transmissive layers of differentthicknesses, and that provides high diffraction efficiency for theincident diffracted light on the information recording face of therespective recording medium (i.e., the light beams are usedefficiently), and that can record or erase information requiring a highpower light beam. Further, with the foregoing configuration, the powerof the respective light beams can be reduced to prevent increase ofpower consumption in the light source.

In the optical pickup of the present invention, the diffraction opticalelement causes the first light beam to be incident on the focusing meansas a diffracted ray of the first order, and causes the second and thirdlight beams to be incident on the focusing means as diffracted rays ofthe zeroth order. Further, the depth of the diffraction grating on thediffracting face of the diffraction optical element is set so that thediffraction optical element has the highest diffraction efficiency forthe first order component of the diffracted light for the first lightbeam, and has the highest diffraction efficiency for the zeroth ordercomponent of the diffracted light for the second and third light beams.

With this configuration, the diffraction optical element causes thefirst light beam to be incident on the focusing means as a diffractedray of the first order, and causes the second and third light beams tobe incident on the focusing means as diffracted rays of the zerothorder. In addition, the depth of the diffraction grating on thediffracting face of the diffraction optical element is set so that thediffraction optical element has the highest diffraction efficiency forthe first order component of the diffracted light yields for the firstlight beam, and has the highest diffraction efficiency for the zerothorder component of the diffracted light for the second and third lightbeams. As a result, a loss of light quantity of incident light on therespective recording media can be reduced for any of the diffractedrays, and the same focusing means can be used to form desirable lightspots on the recording media respectively having light transmissivelayers of different thicknesses. Further, with the foregoingconfiguration, the degrees of divergence with respect to the diffractionoptical element can be reduced for the second and third light beams,making it possible to suppress the adverse effect of radial shifting ofthe focusing lens during tracking or other operations. As a result, anoptical pickup is realized that is compatible with three kinds ofrecording media with light transmissive layers of different thicknesses,and that provides high diffraction efficiency for the incidentdiffracted light on the information recording face of the respectiverecording medium (i.e., the light beams are used efficiently), and thatcan record or erase information requiring a high power light beam.Further, with the foregoing configuration, the power of the respectivelight beams can be reduced to prevent increase of power consumption inthe light source.

In the optical pickup of the present invention, the diffracting face andthe refracting face of the diffraction optical element are a convergingdiffracting face and a concave refracting face, respectively.

With this configuration, the range of available wavelengths can be madewider than that for the optical pickup designated for the firstrecording medium, thereby improving the wavelength dependentcharacteristic over the case of solely using the objective lensdesignated for the first recording medium. Thus, with the foregoingconfiguration, a desirable light focusing characteristic can bemaintained even in the presence of wavelength fluctuations caused by,for example, mode hopping. Further, the minimum pitch on the diffractingface of the diffraction optical element can be increased, making iteasier to fabricate the diffraction optical element.

Further, with the foregoing configuration, the light beams can be madeincident on the focusing means with predetermined degrees of divergenceeven when the incident light beams on the focusing means are weakdiverging rays. Accordingly, the adverse effect of radial shifting ofthe focusing means can be reduced. In addition, the light sources can beprovided at a distant position from the focusing means, allowing thelight sources to be disposed more flexibly.

In the optical pickup of the present invention, the depth of thediffraction grating on the diffracting face of the diffraction opticalelement is set so that the diffraction optical element has a 90% orgreater diffraction efficiency for the first order component of thediffracted light for the first light beam.

With this configuration, the diffraction efficiency can be improved forthe diffracted light of all of the first, second, and third light beams.This enables the power of the respective light beams to be reduced,thereby reducing power consumption of the light source. Further, withthe foregoing configuration, an optical pickup is realized that canprovide high diffraction efficiency for the first light beam, for whichfabrication of a high power laser is difficult.

In the optical pickup of the present invention, the diffraction opticalelement satisfies|Φoutr|>|Φinr|, and |ΦoutIr|>|ΦinIr|where Φinr and ΦinIr are degrees of convergence and/or divergence ofincident rays of the second and third light beams, respectively, on thediffraction optical element, and Φoutr and ΦoutIr are degrees ofconvergence and/or divergence of incident rays of the second and thirdlight beams, respectively, on the focusing means.

With this configuration, the degrees of convergence and/or divergencehave greater absolute values for the incident rays of the second andthird light beams entering the diffraction optical element than for theemergent rays of the second and third light beams leaving thediffraction optical element. This enables the second and third lightbeams to be incident on the diffraction optical element with degrees ofconvergence and/or divergence of small absolute values, whilesuppressing the spherical aberration caused by the thickness differenceof the light transmissive layers. That is, the second and third lightbeams can be made incident as near parallel rays. Thus, with the opticalpickup of the present invention, the third light beam can be incident onthe diffraction optical element with a small degree of convergenceand/or divergence.

In the optical pickup of the present invention, the diffraction opticalelement causes the first, second, and third light beams to be incidenton the focusing means as diffracted rays of the first order, and thedepth of the diffraction grating on the diffracting face of thediffraction optical element is set so that the diffraction opticalelement has the highest diffraction efficiency for the first ordercomponent of the diffracted light for all of the first, second, andthird light beams.

With this configuration, the diffraction optical element causes thefirst, second, and third light beams to be incident on the focusingmeans as diffracted rays of the first order, and the depth of thediffraction grating on the diffracting face of the diffraction opticalelement is set so that the diffraction optical element has the highestdiffraction efficiency for the first order component of the diffractedlight for all of the first, second, and third light beams. As a result,a loss of light quantity of incident light on the respective recordingmedia can be reduced for any of the diffracted rays, and the samefocusing means can be used to form desirable light spots on therecording media respectively having light transmissive layers ofdifferent thicknesses. Further, with the foregoing configuration, thedegrees of divergence with respect to the diffraction optical elementcan be reduced for the second and third light beams, making it possibleto suppress the adverse effect of radial shifting of the focusing lensduring tracking or other operations. As a result, an optical pickup isrealized that is compatible with three kinds of recording media withlight transmissive layers of different thicknesses, and that provideshigh diffraction efficiency for the incident diffracted light on theinformation recording face of the respective recording medium (i.e., thelight beams are used efficiently), and that can record or eraseinformation requiring a high power light beam. Further, with theforegoing configuration, the power of the respective light beams can bereduced to prevent increase of power consumption in the light source.

In the optical pickup of the present invention, the diffracting face andthe refracting face of the diffraction optical element are a divergingdiffracting face and a convex refracting face.

With this configuration, the same focusing means can be used to formdesirable light spots on the recording media respectively having lighttransmissive layers of different thicknesses. Further, with theforegoing configuration, the light beams can be made incident on thefocusing means with predetermined degrees of divergence even when theincident light beams on the focusing means are weak diverging rays.Further, with the foregoing configuration, the degrees of divergencewith respect to the diffraction optical element can be reduced for thesecond and third light beams, making it possible to more effectivelysuppress the adverse effect of radial shifting of the focusing lensduring tracking or other operations. In addition, the light sources canbe provided at a distant position from the focusing means, allowing thelight sources to be disposed more flexibly.

In the optical pickup of the present invention, the refracting face ofthe diffraction optical element is aspherical.

With the foregoing configuration, the spherical aberration caused by thethickness difference of the respective light transmissive layers of therecording media can be reduced more effectively, and impairment ofshifting characteristic of the focusing means can be suppressed. As aresult, a desirable light focusing characteristic can be obtained.

It is preferable in the optical pickup of the present invention havingthe foregoing configurations that the diffracting face of thediffraction optical element is formed on a refracting face.

With this configuration, the refracting face and the diffracting face ofthe diffraction optical element need not be aligned, making it easier tofabricate the diffraction optical element.

It is preferable in the optical pickup of the present invention havingthe foregoing configurations that the diffracting face of thediffraction optical element includes a diffraction grating that isserrated or stepped.

With this configuration, the diffraction optical element can improvediffraction efficiency for the respective light beams. This reduces thepower of the light source of the respective light beam, thereby reducingthe power consumption of the light source. The foregoing configurationis particularly effective for an optical pickup that records or erasesinformation requiring a high power beam.

The present invention provides an optical pickup for focusing first,second, and third light beams of wavelengths λ1, λ2, and λ3, which arerelated to one another by λ1<λ2<λ3, on information recording faces offirst, second, and third recording media respectively having lighttransmissive layers, the light transmissive layers being formed on theinformation recording faces and respectively having thicknesses t1, t2,and t3, which are related to one another by t1=t2<t3, the optical pickupincluding: an objective lens for respectively focusing the first,second, and third light beams on the respective information recordingfaces of the first, second, and third recording media; and a diffractionoptical element, provided on an incident side of the first, second, andthird light beams and provided as an integral unit with the objectivelens, for diffracting and refracting the first, second, and third lightbeams so as to cause the first, second, and third light beams to beincident on the objective lens as diffracted rays of predetermineddiffraction orders, the diffraction optical element causing the thirdlight beam to be focused on the objective lens as a diverging ray, andthe diffraction optical element satisfying|Φin3|<|Φout3|where Φin3 is a degree of convergence or divergence of an incident rayof the third light beam on the diffraction optical element, and Φout3 isa degree of convergence or divergence of an incident ray of the thirdlight beam on the objective lens.

In the configuration of the optical pickup recording or reproducing thefirst, second, and third optical disks using the first, second, andthird light beams of different wavelengths, the objective lens usedcompensates for the aberration that is caused when the first light beamof the shortest wavelength is focused on the first recording medium.

Using the objective lens to focus the second and third light beams onthe second and third recording media respectively having lighttransmissive layers of different thicknesses from that of the firstrecording medium and using different wavelengths from that of the firstrecording medium increases chromatic aberration or spherical aberrationin the second and third light beams. Such an increase of sphericalaberration can be suppressed by compensating for the aberration bygenerating aberration of the opposite direction. This can be carried outby causing the second and third light beams to be incident on theobjective lens as diverging rays.

Here, in order to sufficiently reduce the spherical aberration, thethird light beams must enter the objective lens with a large degree ofdivergence. However, increasing the degree of divergence for theincident light beam on the objective lens increases the coma aberrationthat affects the aperture spot on the recording medium, when theobjective lens moves in the radial direction (direction substantiallyorthogonal to the optical axes of the first, second, and third lightbeams incident on the objective lens) during tracking or otheroperations, with the result that the light focusing characteristic isgreatly impaired.

To avoid this problem, the foregoing configuration uses the diffractionoptical element that is movable with the objective lens, so as to causethe third light beam to be incident on the objective lens as a divergingray. The diffraction optical element functions to satisfy|ΦinIr|<|ΦoutIr|.That is, the degree of convergence and/or divergence has a largerabsolute value for the third light beam emerging from the diffractionoptical element than for the third light beam incident on thediffraction optical element.

This enables the third light beam to be incident on the unit made up ofthe objective lens and the diffraction optical element (objective lensunit) with a degree of convergence and/or divergence of a small absolutevalue. That is, the third light beam can be made incident as a nearparallel ray. As a result, the foregoing configuration is able tosuppress impairment of the light focusing characteristic caused byradial shifting (objective shifting) of the objective lens unit, moreeffectively than the configuration without the diffraction opticalelement.

Thus, with the foregoing configuration, a single objective lens is usedto form desirable light spots on the recording media with lighttransmissive layers of different thicknesses, so as to record orreproduce information. In addition, the light focusing characteristic isnot severely impaired even when the objective lens unit shifts in theradial direction.

It is preferable in the optical pickup of the present invention that thediffraction optical element according to the foregoing configurationcauses the first light beam to be incident on the objective lens as aparallel ray.

With this configuration, the first light beam of a short wavelength,which requires the most accurate light focusing characteristic, isincident on the objective lens as a parallel ray. This suppresses theaberration caused by misalignment of the diffraction optical elementwith the objective lens.

It is preferable in the optical pickup of the present invention that thefirst light beam according to the foregoing configuration is incident onthe diffraction optical element as a parallel ray or a converging ray.

With this configuration, the third light beam can be made incident onthe diffraction optical element with a degree of convergence and/ordivergence of a relatively small absolute value. As a result, impairmentof the light focusing characteristic caused by radial shifting of theobjective lens unit can be suppressed more effectively.

It is preferable in the optical pickup of the present invention havingthe foregoing configuration that the diffraction optical element causesthe first light beam to be incident on the objective lens as adiffracted ray of the second order, and causes the second and thirdlight beams to be incident on the objective lens as diffracted rays ofthe first order, the diffraction optical element having a higherdiffraction efficiency for the second order component of the diffractedlight of the first light beam than for the diffracted rays of any otherdiffraction orders of the first light beam, the diffraction opticalelement having a higher diffraction efficiency for the first ordercomponent of the diffracted light of the second light beam than for thediffracted rays of any other diffraction orders of the second lightbeam, and the diffraction optical element having a higher diffractionefficiency for the first order component of the diffracted light of thethird light beam than for the diffracted rays of any other diffractionorders of the third light beam.

With this configuration, the diffraction efficiency can be improved forall of the first, second, and third light beams. This reduces the powerof the light sources for the respective light beams, thereby reducingthe power consumption of the light sources. The foregoing configurationis particularly effective in an optical pickup that records or erasesinformation requiring a high power beam. Preferably, the diffractionoptical element is designed to have a 90% or greater diffractionefficiency for the second order component of the diffracted light forthe first light beam.

It is preferable in the optical pickup of the present invention that thediffraction optical element according to the foregoing configurationincludes a converging diffracting face and a concave refracting face.

With this configuration, the range of available wavelengths can be madewider than that for the optical pickup designated for the firstrecording medium, thereby improving the wavelength dependentcharacteristic over the case of solely using the objective lensdesignated for the first recording medium. Thus, with the foregoingconfiguration, a desirable light focusing characteristic can bemaintained even in the presence of wavelength fluctuations caused by,for example, mode hopping. Further, the minimum pitch on the diffractingface of the diffraction optical element can be increased, making iteasier to fabricate the diffraction optical element.

The invention being thus described, it will be obvious that the same waymay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. An optical pickup for recording or reproducing information withrespect to (a) a first recording medium having a light transmissivelayer of a thickness t1 on an information recording face and (b) asecond recording medium having a light transmissive layer of a thicknesst2 greater than t1 on an information recording face, the optical pickuprecording or reproducing information with respect to the first recordingmedium by forming a first light spot on the information recording faceof the first recording medium by focusing a first light beam of awavelength λ1 on the information recording face, and the optical pick uprecording or reproducing information with respect to the secondrecording medium by forming a second light spot on the informationrecording face of the second recording medium by focusing a second lightbeam of a wavelength λ2 greater than λ1 on the information recordingface, said optical pickup comprising: a diffraction optical element fordiffracting and refracting incident rays of the first and second lightbeams so as to emit the first and second light beams, the diffractionoptical element including a converging diffraction grating and adiverging lens; and an objective lens for causing respective diffractedrays of predetermined diffraction orders of the first and second lightbeams emitted from the diffraction optical element to focus on therespective information recording faces of the first and second recordingmedia so as to form the first and second light spots, the first lightbeam and the second light beam being incident on the diffraction opticalelement as light beams with different degrees of convergence ordivergence.
 2. The optical pickup as set forth in claim 1, wherein thefirst light beam is incident on the diffraction optical element as aconverging ray, and the second light beam is incident on the diffractionoptical element as a diverging ray.
 3. The optical pickup as set forthin claim 2, wherein the objective lens causes a second order diffractedray of the first light beam emitted from the diffraction optical elementand a first order diffracted ray of the second light beam emitted fromthe diffraction optical element to focus on the respective informationrecording faces of the first and second recording media so as to formthe first and second light spots.
 4. The optical pickup as set forth inclaim 2, wherein the objective lens causes a third order diffracted rayof the first light beam emitted from the diffraction optical element anda second order diffracted ray of the second light beam emitted from thediffraction optical element to focus on the respective informationrecording faces of the first and second recording media so as to formthe first and second light spots.
 5. The optical pickup as set forth inclaim 1, wherein the objective lens causes a second order diffracted rayof the first light beam emitted from the diffraction optical element anda first order diffracted ray of the second light beam emitted from thediffraction optical element to focus on the respective informationrecording faces of the first and second recording media so as to formthe first and second light spots.
 6. The optical pickup as set forth inclaim 1, wherein the objective lens causes a third order diffracted rayof the first light beam emitted from the diffraction optical element anda second order diffracted ray of the second light beam emitted from thediffraction optical element to focus on the respective informationrecording faces of the first and second recording media so as to formthe first and second light spots.
 7. The optical pickup as set forth inclaim 1, wherein the diverging lens is located on a surface, of thediffraction optical element, which faces the objective lens, and theconverging diffraction grating is located on a surface, of thediffraction optical element, which is opposite the objective lens. 8.The optical pickup as set forth in claim 7, wherein the objective lenscauses a second order diffracted ray of the first light beam emittedfrom the diffraction optical element and a first order diffracted ray ofthe second light beam emitted from the diffraction optical element tofocus on the respective information recording faces of the first andsecond recording media so as to form the first and second light spots.9. The optical pickup as set forth in claim 7, wherein the objectivelens causes a third order diffracted ray of the first light beam emittedfrom the diffraction optical element and a second order diffracted rayof the second light beam emitted from the diffraction optical element tofocus on the respective information recording faces of the first andsecond recording media so as to form the first and second light spots.10. The optical pickup as set forth in claim 1, wherein the diverginglens is located on a surface, of the diffraction optical element, whichfaces the objective lens, and the converging diffraction grating islocated on a surface of the objective lens.
 11. The optical pickup asset forth in claim 10, wherein the objective lens causes a second orderdiffracted ray of the first light beam emitted from the diffractionoptical element and a first order diffracted ray of the second lightbeam emitted from the diffraction optical element to focus on therespective information recording faces of the first and second recordingmedia so as to form the first and second light spots.
 12. The opticalpickup as set forth in claim 10, wherein the objective lens causes athird order diffracted ray of the first light beam emitted from thediffraction optical element and a second order diffracted ray of thesecond light beam emitted from the diffraction optical element to focuson the respective information recording faces of the first and secondrecording media so as to form the first and second light spots.